New in Star Trek: Discovery Season 5 is Rayner, a Kellerun Starfleet veteran who started the season captaining the USS Antares -- then nearly got early retirement after brashly endangering thousands. Given a second chance by the once-rebellious Captain Michael Burnham as her new first officer on the USS Discovery, Rayner's curt and no-nonsense approach clashes with the rest of the crew. And with the fate of the entire galaxy hanging in the balance, Rayner will have to learn how to work with them while they, in turn, learn new tricks of the trade from him.

In an interview with CBR, Star Trek: Discovery actor Callum Keith Rennie talks about joining the cast for the Paramount+ drama's final season. He also teases some of Rayner's backstory and explains how his character fits into the show overall. Plus, he reflects on his own lengthy list of sci-fi series credits.

A lot of your scenes in Star Trek: Discovery Season 5, Episode 3, "Jinaal," are opposite Mary Wiseman, and it's fun seeing how bubbly Tilly is juxtaposed with how taciturn Rayner can be. How did you like working with Mary?

Mary is wonderful. I love scenes with her; she's just great. There is that thing, going from fun to stoic Rayner-ville. They're nice bits!

When you were approaching the part of Rayner, was there any line in the script or character description that stood out to you?

It was more because there was a whole bunch of action. I went "That's great! I get to ride a space snowmobile thing? Yes!" It was that he was an action guy and a ship commander -- that was what appealed to me.

You've gotten to be a part of major sci-fi properties like the memorable reboot of Battlestar Galactica and The X-Files. How would you describe getting to be part of Star Trek, and where does Star Trek stand in your own appreciation of the genre?

Things kind of layer in later. You do it and then it sits. I find you do a project, you look at it, and you'll have misgivings about it, missed opportunities or if you should've done this or that. There is so much of that, that it's usually in time that an appreciation of what that was, how that felt and your own personal reaction to it changes. I can easily forget all of the other work that I've ever done – I'm not sure what that is. I have to look at something and go "Oh yeah! That was great!" Maybe that's always moving onto the next thing or the future, rather than being stuck in the past or giving myself a pat on the back.

Those shows have meant more to me through time than they did immediately out of the gate. Star Trek will fall into that place; [right] now, it's really recent. I was talking to someone the other day and said "I think I did Californication. I'm not sure if I did any interviews for it at all…" There are some things you just do and it lives its life. I was working on Discovery, forgetting the immense fan following and the history of it, forgetting all of that stuff, and now we're here, and I'm remembering this whole world of 60 years, and now I'm part of that. It's very nice to be on board.

Admiral Vance makes references to being friends with Rayner for over 30 years. How much of Rayner's backstory was given to you, and how much did you develop yourself?

There wasn't all that much backstory in the sense of playability. There was only his history through the Burn and conflicts at other times, which may have molded who he was as a young Kellerun. He is holding a type of anger about specific things and specific unfinished business, which has carried on through his career.

Trying to Google and get into the history of the Kelleruns, there wasn't much to go on, so it was like "I guess whatever I do will be Kellerun enough!" There wasn't tons, but as the episodes go on, you'll go "Oh, that's why he has such a chip on his shoulder that may have held him in a non-developing place."

What do you think Rayner brings to the Discovery as its new first officer, and what do you think he brings to Star Trek: Discovery in general?

I think he brings conflict and another point-of-view. He brings history and a style that’s not as different from Burnham’s as they like to think, but it’s more ingrained. There’s a gravity to it because, when he comes in, his wings have been clipped. He’s doing that just because he likes Starfleet, and he doesn’t know what else to do and this is better than retirement for him.

He also doesn’t know how he’s going to do it, based on him going “I don’t know how to be second-in-command. I don’t know how to do that. Maybe I can learn. The worst that can happen is they can fire me again or send me out to pasture. I’ll try it!” He brings all of his stuff, the good, the bad and the ugly. [He learns] from [Burnham] and she learns from [him]. Some of the stuff [Rayner does] is impulsive and doesn’t have a lot of breaking off into discussion groups to get it done. He just goes and does it on his own.

You mentioned the action in Discovery standing out to you. How did you want to accentuate that physicality with your performance, and what is the personal appeal of getting to do action for you?

For this particular project, I only had one script to go on, and it was us down on the planet doing a bunch of stuff. [The appeal] is that you're just not a talking head. You get to move around and do stuff. For me, if I get to punch something in the head, I'm still in the game somehow. [Laughs.] I still get to do action stuff, even though I'm getting on a bit.

Callum, what else can you tease for Rayner as fans progress deeper into Star Trek: Discovery Season 5?

Nothing; you've got to live and learn it. I watched Episodes 3 and 4 last night and I got a nice reminder of all that stuff. Episode 4 is so great. On first viewing, there was a distance. As I was watching these last night, [I] was like "Oh, I like this! These are really good!" I can't remember that far ahead. There is a moment he gets to, which is a little bit of the old days -- which is exactly what Discovery needs: someone who is a little bit of chaos.

Created by Bryan Fuller and Alex Kurtzman, Star Trek: Discovery releases new episodes Thursdays on Paramount+.