The X-Files is a sci-fi drama series starring Gillian Anderson as the skeptical F.B.I. Agent Dana Scully and David Duchovny as F.B.I. Agent Fox Mulder, a believer in the paranormal. The agents are assigned to a department of the F.B.I. known as the X-Files, where they investigate cases involving inexplicable phenomena. Agent Scully is a medical doctor who approaches situations with science-based facts, while Agent Mulder is a walking encyclopedia of The Unexplained. Together, the agents work to find the truth, though unknown forces keep them shrouded in darkness.

The series rotates between an ongoing mythology arc and Monster of the Week episodes, which provides endless opportunities for guest stars. Since the show's debut in 1993, illustrious stars have significantly contributed to stand-alone episodes. While there are too many to name, some of The X-Files' guest stars provided significant performances and added luster to the beloved series' glow. Here are the stars that have shined the brightest in the vast unknown.

10 Téa Leoni and Garry Shandling

Season 7, Episode 19, "Hollywood A.D."

A.D. Skinner's (Mitch Pileggi) college pal, Hollywood screenwriter Wayne Federman (as himself), is given an all-access F.B.I. pass to observe Mulder and Scully as they investigate an X-File. Federman parlays his experiences with the agents and their paranormal investigation into a "loosely based" sensational big-screen adaptation and invites them to attend. Though Mulder requests that Richard Gere portray him in the film, Federman informs him that Gere has been cast as Skinner instead. Agents Mulder and Scully arrive in Hollywood and meet their acting counterparts, Garry Shandling and Téa Leoni, who present them with fascinating character study questions.

In Duchovny's second foray as a writer on the series, the actor successfully produced one of the few comedic episodes in The X-Files history. The episode is littered with self-referential deprecations and playful exchanges between the cast, a welcome reprieve from the persistent dark shroud of conspiracy and pulse-pounding momentum in the seventh season. Adding to the meta atmosphere, Duchovny cast his then-wife, Leoni, as Scully in the fictional film adaptation of The X-Files. Fans of The Garry Shandling Show might've noticed Duchovny's subtle callback referencing a Shandling/Duchovny "attraction" when Scully tells Mulder she thinks "Garry Shandling has a crush" on him after the agents visit the set. Leoni requires an on-set tutorial for running outfitted with Scully's heels, while Shandling's inquiries into Mulder's "habits" are...anatomically specific. It's a hysterical episode featuring a bubblier-than-average Skinner.

9 Luke Wilson

Season 5, Episode 12, "Bad Blood"

The agents head to rural Texas to investigate evidence of exsanguination in "Bad Blood," one of the funniest, highest-rated episodes in the series. Scully and Mulder are independently drugged by the town's inhabitants, resulting in confusion and uncertainty regarding events that may or may not have occurred. Before submitting their case report to Skinner, the agents share their riotous versions of events to get their stories straight. Mulder is adamant in asserting that a recreational vehicle collective of vampires duped them. Enamored with town sheriff Hartwell (Luke Wilson), Scully maintains that there must be a plausible explanation for all vampiric behaviors, though she has zero theories.

In an iconic episode written by the man with the golden pen, Vince Gilligan, fans of the beloved series witnessed the comedic capacity of actors Anderson and Duchovny. While the pair of detectives (in rare, spooftastic splendor) were the main draw, "Bad Blood" was further enhanced by Wilson's appearance and prosthetic prowess. The mild-mannered actor fits seamlessly into the episode as the duplicitous, handsome hillbilly Sheriff Hartwell. Audiences giggled at Scully's exaggerated lust for Wilson's handsome "aw, shucks" routine and belly-laughed at Mulder's buck-toothed, dim-witted description of the Sheriff. In the end, Sheriff Hartwell proved to be none of those things in Wilson's capable hands.

8 Lily Tomlin and Ed Asner

Season 6, Episode 6, "How the Ghosts Stole Christmas"

It's Christmas Eve, and Mulder has summoned Scully to investigate a haunted house buzzing with paranormal activity. The last thing Scully wants to do is wander the halls of an abandoned mansion on Christmas Eve, so she tries to bail, but Mulder has stolen her car keys and disappeared into the house. Mulder informs Scully that the couple who inhabited the home circa 1917 died in a murder-suicide lover's pact and returned to haunt the house every Christmas Eve. Scully's annoyance and exasperation turn to panic as the agents become pawns in the ghost's annual game with deadly consequences.

In the unforgettable spooky holiday episode, Lily Tomlin and Ed Asner guest star as ghost couple Maurice and Lyda. The comedy legends get ghoulish in a rare opportunity to play against type (okay, they're not that scary) as the couple who nearly succeeds at turning Mulder and Scully against each other. The episode became an instant classic among fans of the show and has been considered one of the best holiday episodes of any series by many critics. A longtime X-Phile herself, Tomlin had previously approached the show's producers with guest starring interest, resulting in a part tailor-made by series creator Chris Carter. "How the Ghosts Stole Christmas" could have gone darker, but the episode's undeniable romantic current was a gift Scully and Mulder shippers were dying to unwrap.

7 John Hawkes

Season 6, Episode 18, "Milagro"

Mulder's mysterious new neighbor, Phillip Padgett (John Hawkes), is working on a novel about a killer who rips the still-beating hearts from the chests of his victims. Padgett has a wordless encounter with Scully while visiting Mulder's apartment, prompting the author's fascination with her. The author professes to know everything about Scully, and after following her into a church, he rattles the agent by describing her in intimate detail. Meanwhile, Padgett's fictional character, Naciamento (Nestor Serrano), murders people in real time as the author depicts them on the page. Mulder and Scully suspect the novelist is responsible for the murders, but Padgett has piqued Scully's curiosity with his unnervingly accurate assessment of the lonely agent. Her uncharacteristic absence of caution and desire to be seen becomes a dangerous game of the heart.

While critics were split regarding the episode's premise, they couldn't deny Hawkes' creepy consideration of Phillip Padgett. The X-Files introduced a colorful Rolodex of disturbing admirers during its run, but Padgett's observant ogling and quiet, yet passionate pursuit of Scully stood out. With the use of voiceover, Hawkes's presence loomed, his words insidiously enveloping an unsuspecting Scully. The vulnerable agent had sustained an alien abduction and was held captive by a murderer, but Padgett's invasive infiltration into Scully's psyche was somehow more frightening. Donnie Pfaster was terrifying, and Duane Barry was unhinged, but Hawkes' portrayal of the soft-spoken, diligent writer seared into viewer's memories in perpetuity.

6 Joe Morton

Season 8, Episode 6, "Redrum"

A gifted prosecutor wakes up in a prison cell wearing an orange jumpsuit. He has a fresh gash on his cheekbone and no memory of how he got there. The baffled attorney is identified as Martin Wells (Joe Morton), awaiting trial for the murder of his wife the previous day. As Martin departs the cell en route to a transfer, he is shot by an angry man among a crowd of reporters and spectators. Martin wakes the next day in his original cell, but the gash on his cheek has disappeared. He remembers being shot but has no wound. Agents Scully and Doggett (Robert Patrick) arrive, further compounding Martin's feeling of déjà vu. The following day begins the same way, and Martin understands that time is moving in reverse, providing a narrow window to alter the past and save his wife.

"Redrum" is a Joe Morton showcase. The versatile and decorated character actor has appeared in over 70 films and legendary T.V. series, including an Emmy-winning role for his work on Scandal. Morton's commanding, believable depiction of a desperate man navigating borrowed (reversed) time on The X-Files should've earned the gifted actor a second Primetime Emmy. His character grapples with bewilderment and disbelief at his incarcerated predicament with mounting pressure as the clock ticks backward. Facts are simultaneously revealed to audiences alongside the anguished prosecutor, adding to the episode's tension. In the style of Groundhog's Day sans humor sprinkled with a home invasion, Morton left his mark on a season fans of the show mainly considered unremarkable.

5 Harriet Sansom Harris

Season 1, Episode 11, "Eve"

Mulder and Scully investigate a pair of identical murders that appear to have been committed at the same time, despite their lack of proximity. Evidence from each crime scene reveals uncannily similar clues, including 8-year-old carbon-copied witnesses. With the help of Mulder's informant, Deep Throat (Jerry Hardin), the agents travel to a hospital housing the criminally insane. They visit a volatile patient secured with restraints named Eve 6 (Harriet Sansom Harris), who explains that she, along with many others, is a product of a genetic cloning experiment called The Litchfield Project. The clones, all named Adam or Eve, were born with extra "special" chromosomes. However, their advanced "abilities" came with unsavory components like homicidal rage and varying degrees of psychosis.

In one of the more unsettling installments of The X-Files, identical twins (Erika and Sabrina Krievins) with flat affect could have fulfilled the episodic creep requirement (see The Shining). Fortunately for fans, Harris was cast as Eve 6, 7, and 8, aka Sally Kendrick, raising the scary stakes. Harris, a Julliard-educated theater actor with scores of T.V. and film experience, was kind enough to lend her bone-chilling talent. Critics and creator Carter praised Harris' embodiment of three distinct iterations of Eve, and viewers are forever haunted by her terrifying guard-biting rendition of Eve 6, especially. There have been countless Monster of the Week instances of nightmare fuel, but Harris' contribution to "Eve" is marquee fright. In a deliberate act of violence, the episode ends with a shot of Eve 7, free to move about the cabin. Maybe we haven't seen the last of The Litchfield Project.

4 Bryan Cranston

Season 6, Episode 2, "Drive"

In a speeding vehicle, Patrick Crump (Bryan Cranston) races to save his wife, whose increasing cranial pressure appears to lessen as they travel westward. The police begin to give chase, assuming the bizarre scene is a kidnapping in progress. Police stop Crump, and his wife dies. Mulder and Scully learn of the situation unfolding on live T.V., and Mulder drives to intercept Crump. Meanwhile, Scully investigates the area surrounding Crump's home and finds birds and a dog that has succumbed to the same fate as Mrs. Crump. Further examination reveals a nearby U.S. Navy antenna broadcasting E.L.F. waves and a deaf neighbor exhibiting no sign of distress or head pain. Desperate to keep moving west and suspicious of the government, Crump pulls Mulder's weapon and directs him toward the Pacific before time runs out.

Heralded by fans and critics as one of the best episodes of The X-Files, "Drive" never takes its foot off the gas. Some compared the non-stop frenetic premise of the episode to the movie Speed with a sci-fi twist. In addition to giving fans one of the most exciting hours in series history, "Drive" was the soil that nurtured the seeds of one of the most prolific T.V. series of all time, Breaking Bad. After working with Cranston on the episode, X-Files writer Vince Gilligan found Walter White. Through Cranston's anguished, paranoid performance as Patrick Crump, viewers were taken along for the ride. Fans looking for white-knuckled anxiety look no further than "Drive" -- T.V. can't lose with Cranston behind the wheel.

3 Michael McKean

Season 6, Episode 4, "Dreamland, Part I & II"

It's Freaky Friday meets The X-Files in the only two-part non-mythology episodes of the series, "Dreamland I" and "Dreamland II." Mulder and Scully are poking around Area 51 when a cavalcade of vehicles carrying soldiers suddenly surrounds them. A man in black, Morris Fletcher (Michael McKean), steps out of a car and meets Mulder in the street just as a bright object flies overhead. When the light disappears, Mulder realizes they have switched bodies, but Morris doesn't notice. Mulder, in Morris's body, is returned to the Fletcher family home in Nevada. Meanwhile, Morris, in Mulder's body, is thrilled to be rid of his nagging wife and ungrateful children. Try as he might, Mulder can't seem to sell Scully on the body-swapping situation, to Morris's delight.

Comedic actor McKean has made audiences laugh for decades, beginning with Laverne & Shirley in 1976. From Saturday Night Live to his prominent parody presence in movies made by Christopher Guest, McKean has mastered comedic timing. It was unsurprising when his guest spot on a thrilling, famously dark sci-fi series elicited fits of laughter from viewers. One of many highlights occurs when the bodily displaced pair perform a synchronized mirror dance wearing only their undergarments. Infiltrating Mulder's life, Morris takes every (hilarious) opportunity to make a pass at Scully, resulting in some of the red-haired agent's funniest retorts ("Mulder, you are acting bizarre"). McKean's turn as the womanizing, selfish Morris was so good that the character returned in later episodes, "Three of a Kind" and "Jump the Shark." While critics favored "Dreamland, Part I" over its continuation, McKean (and everything else) is worth your time.

2 Brad Dourif

Season 1, Episode 13, "Beyond The Sea"

Scully wakes to find her father seated in her apartment, silently uttering something she can't understand. The phone rings, startling her, and as she looks back at her visiting father, he isn't there. On the phone, Scully's mother shares news of her father's death. Bereft, Scully rallies and accompanies Mulder to interview a convicted murderer on death row claiming to have information related to a missing couple. Luther Lee Boggs (Brad Dourif) is scheduled to be executed but claims that his visions could lead the agents to the kidnapper's hideout. Mulder is uncharacteristically skeptical of Boggs and discounts the visions as a last-ditch effort to commute his sentence. Scully is prepared to adopt Mulder's stance until Boggs uses her father's voice to speak to her.

Until "Clyde Bruckman's Final Repose" aired in season 3, audiences would be hard-pressed to compare Brad Dourif's powerful performance as Luther Lee Boggs to any guest. Anderson is also remarkable in the episode, particularly in scenes where Scully is one-on-one with Boggs, some of her most vulnerable moments in the series. To explain his visions, Dourif acts as a conduit for physical torment. Viewers are conflicted as Boggs recounts the pain of his victims coursing through his body in a tearful, agonizing display. Boggs is a serial killer on death row, yet with Dourif's convincing anguish and emotionally exhausting admission, there is a moment of room for sympathy. "Beyond the Sea" is a pivotal piece of Scully's story, but if not for the generous participation of Dourif, it wouldn't pack such an enduring punch.

1 Peter Boyle

Season 3, Episode 4, "Clyde Bruckman's Final Repose"

A serial murderer of psychics is one step ahead of Mulder and Scully until they employ the aid of insurance salesman Clyde Bruckman (Peter Boyle). Bruckman has a valuable gift: he sees how people die. The lonely old man with sustained depression and visions of strangers' deaths gets closer to the killer of clairvoyants while his mortality begins to loom large. As their investigation progresses, Bruckman entertains and offers morsels of insight into Mulder's end (he asked) while an unmistakable connection with the killer grows. The psychic insurance salesman becomes fond of Mulder and Scully, which Scully especially reciprocates. When a vision of Mulder falling by the knife blade of the elusive killer, Bruckman prepares for his curtain call.

"Clyde Bruckman's Final Repose" is one of, if not the most beloved episodes of the series because of one man: Peter Boyle. By season 3, audiences have fallen hard for Mulder and Scully, so their heartbreak for Clyde Bruckman smarts like a compound wallop. While there are poignant scenes laced with ruminations on life and the monotony of existence, Boyle's involvement serves heaviness with an air-whipped side of humor. Fans of the show weren't the only ones praising Boyle's performance; the actor won an Emmy Award for Outstanding Guest Actor in a Drama Series. In a series spanning 24 years of improbable, revelatory, and hysterical characters, Boyle's work as Bruckman (along with Darin Morgan's skilled verse) transcended everything. X-Philes will be forever grateful for the most excellent response to the Scully-posed question: "All right, so how do I die?" Clyde Bruckman: "You don't."

The X-Files can be streamed on Hulu in the U.S.

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