On the show, Rebecca plays the matriarch of the Pearson clan and she's often shown in flashback sequences in which she raises her triplets with Milo Ventimiglia, he plays her husband Jack Pearson on the show.
"I mean, we get to hold these newborn children, and I can't believe their parents were willing to hand over their babies to me," she told Conan O'Brien. "I'm like, 'Hi, I'm Mandy. I have absolutely no experience with small children,' and they're like, 'Here's my 2-week-old baby.'"
Before the show, she said she had no experience changing a diaper and had crew members showing her how it's done.
"It has sort of activated that maternal side of me that I'm totally ready to have kids now," she said.
During the majority of her on-screen time, Mandy shares the screen with child actors Mackenzie Hancsicsak, Parker Bates and Lonnie Chavis, who portray Pearson kids Kate, Kevin and Randall, respectively.
"We have three extraordinary young actors on the show, who have fantastic parents by the way. They're very attentive," Mandy said. "I think it's a lot of their first jobs for most of them, and anytime there's a scene that requires them to eat, I have to remind them — Milo, who's my husband on the show — we're always like, 'Guys, slow down because you guys don't realize you're committing yourself to eating that same amount for the entirety of the scene, which can sometimes be a couple hours.'"
Her newfound maternal instinct even kicked in during a recent episode when she had to tell the kids to not eat the set decor.
"I remember we were shooting the Christmas episode, and there was some melted peppermints in a candy dish that wasn't even from the prop department," she said. "Clearly, it was set decoration, and I turn my back for one second, and I turn back, and all of them have jammed peppermints in their mouths."