How did 'Big Bang Theory' star pull off Comic-Con surprise?

The Big Bang Theory panel at Comic-Con started out routine enough. All the writers and producers filed out onstage, and the only star in sight was Melissa Rauch, who plays Bernadette on the show and moderated Friday's panel.

But when a "fan" approached the mic in Princess Leia's Return of the Jedi disguise, the mask was removed and BBT leading man Johnny Galecki was revealed. So how did he pull off such a big surprise? And why the Leia costume ("Not the bikini. I wore that underneath," he quipped)?

EW's Sandra Gonzalez gets those answers and a tease of what's coming next in season 7 in the video below: