Kaley Cuoco talked about how she dated Johnny Galecki for two years while they were costars. Their characters, Penny and Leonard also dated on the show. They kept the relationship absolutely private but eventually ended things, Kaley Cuoco said, “I don’t want to be hiding. We couldn’t do anything. It wasn’t as fun as we wanted it to be.”
Mayim Bialik who plays Amy on the show learned to play the harp because it is Amy’s instrument of choice. Jim Parson’s also learned how to play the theremin for his character, Sheldon. A theremin is an early electronic musical instrument, the perfect choice of instrument for Sheldon.
Melissa Rauch who plays Bernadette does not actually talk in such a high-pitched voice. She based the voice for her character on her mother’s voice, “only without the New Jersey!”
There is only one set of stairs leading up to Penny, Leonard, and Sheldon’s apartments. The scenes where the actors walk up or down the three flights of stairs have to be shot in separate takes because the actors and the scenery needs to be reset after each flight.
Stephen Engel was the one who started “Bazinga!” Sheldon’s catchphrase when he does or says something he means to be humorous is “Bazinga!” It all began when Stephen Engel would play practical jokes in the writers’ room, saying “Bazinga” instead of “gotcha.”
In an unaired first pilot, Amanda Walsh plays Penny’s character, at the time named Katie. Katie is more sexual and raunchy in this pilot than Penny is after Kaley Cuoco took on the character. The test audience did not like Amanda Walsh’s version of Penny, feeling overprotective of the guys.
“Soft Kitty” is a adapted version of “Warm Kitty,” a children’s song by Edith Newlin. Bill Prady came up with the adaptation of this song that sooths Sheldon throughout the show. A teacher from his daughter’s preschool had heard it in Australia and taught the kids.