'Big Bang Theory' cast goes to prom! See their old high school photos

This will be the first time the group has been back up to the roof since Howard (Simon Helberg) and Bernadette (Rauch) tied the knot!

For all you Shamy fans, Molaro also teases that tonight is a big step forward in Sheldon (Jim Parsons) and Amy's (Bialik) relationship. Could this next step finally mean coitus?

"That's one of the secrets I'm going to keep," Molara says.

VIDEO: Three Set Secrets of 'Big Bang'

"I think we're as together as we need to be right now," Bialik told ET. "We sort of keep riding that wave of enjoying each other the way we are and wanting more, but accepting the other person where they are. I think it's really sweet like this."

While Sheldon and Amy steer clear of the bedroom, are Penny (Kaley Cuoco-Sweeting) and Leonard (Johnny Galecki)avoiding the altar?

"I don't see it happening soon since considering it took them seven years to get engaged," Cuoco-Sweeting says.

"There's a whole new world of problems now," Galecki adds. "If there's anyone that's been engaged before knows, that doesn't solve everything between a couple."

Watch the full video for a peek at tonight's prom-themed episode and to see the cast's reaction when we showed them their old high school photos