Little House on the Prairie’s Alison Arngrim and Melissa Gilbert Were Close Despite Playing ‘Mortal Enemies’ (Exclusive)

Life didn't imitate art for Little House on the Prairie stars Melissa Gilbert and Alison Arngrim.

During an exclusive interview with PEOPLE at the classic family drama's 50th Anniversary Cast Reunion and Festival in Simi Valley, California, on Saturday, Arngrim — who played schoolyard bully Nellie Oleson on all nine seasons of the show — reflected on her relationship with Gilbert, 59, who starred as Laura Ingalls Wilder.

"Here's Melissa Gilbert and I playing mortal enemies, beating each other senseless all week," says Arngrim, 62. "And then on the weekends, we'd go to each other's house for a slumber party and we were hanging out."

Arngrim says she and Gilbert were "like sisters" when the cameras stopped rolling, and were even choreographing their fight scenes together from the first year they began working together in 1973.

"It's so completely bonkers," Arngrim jokes.

She says that fans often ask her to sign a particular photo of her and Gilbert posing together behind the scenes while filming a Christmas episode in which their characters fight — and express shock that the young ladies are smiling in the photo.

"People say, 'It's little Nellie and Laura, but you're smiling. The two of you don't smile at each other in that episode,' " she recalls. "I'm like, 'This was us for real.' "

Although Arngrim and Gilbert were able to separate reality from fiction, Nellie Oleson became such an iconic bully that viewers weren't able to do the same.

"People hated me. They hated me, hated me," Arngrim says, remembering that someone once threw a cup of orange soda in her face while at a Christmas parade.

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"I was a moving target and they hit me. So I'm kind of impressed," the actress and comedian tells PEOPLE. "But what struck me about it, and all the times that people attack me, I was like, 'What am I doing that they are so convinced this is real, that they're just losing it?' "

Arngrim says she's learned to embrace being known as a bully, and shared her experiences on the show in a live one-woman show she did called Confessions of a Prairie Bitch.