They arrive a couple of hours before show time to slip into the shoes of characters readers first became acquainted with in 1960.
"I have to get my bow tie on, I have to make sure my caller is fixed, I have to get my undershirt on, make sure my buttons are buttoned to my pants," Charlie Mathis explains.
He plays Dill
"If you know me, he's pretty much like me, he's sort of solemn, he can make people laugh."
Ronan Ryan plays Jem and his twin sister Kaylee plays Scout.
"I've always loved the book, it's a classic piece of literature and I think being able to play the part is even better," Kaylee says.
Together they're bringing the pages of "To Kill a Mockingbird" to life on stage.
It's a plot that's a bit heavy for a 9 year old, but something these 13 year olds can certainly appreciate.
"I like the meaning behind it," Ronan says.
"Doing the show and being scout helps me really understand what was going on at that time," Kaylee adds.
They are talented St. Louis kids who beat out a couple dozen kids for their roles.
"Well when I first practiced my lines I thought, this might be easy cause I only had a few lines to practice. And then my mom told me it's at The Rep and I'm like woah 35 shows, practice every night that's really big," Charlie says.
All three obviously have what it takes, but what's next, well they're not exactly sure.
"I have a lot of other careers I have in mind. I think of myself as more in the arts category so I like pretty much anything in that area," Kaylee points out.
For now they're just happy to be staying up past their bedtime.