Kaley Cuoco In A ‘Sexy Cat’ Costume Helped Ignite A Real-Life Spark Between Her And ‘Big Bang Theory’ Co-Star Johnny Galecki

The Big Bang Theory, the inspiration for Nandor’s favorite slot machine, premiered 15 years ago last month. In honor of the occasion, author Jessica Radloff wrote an oral history, The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series, in which she spoke to the show’s producers and cast, including Kaley Cuoco and Johnny Galecki.

It was known that they dated from 2008 to 2010, not unlike their fictional Big Bang counterparts, Penny and Leonard, but they didn’t discuss the relationship with any depth until speaking to Radloff for the book. “I had a very big crush on Johnny early on. I was so not even hiding it. He has such swagger. We were both dating people at the time, but I only had eyes for Johnny,” Cuoco said in an excerpt published by Vanity Fair.

The first time Penny and Leonard kissed in a season one episode was also the first time Cuoco and Galecki locked lips. “I knew I looked real cute in that outfit, so I was feeling really good about myself,” the Harley Quinn star said (she wore a “sexy cat” costume in the Halloween episode). “And I knew he thought I looked cute because he commented on how cute I looked in that outfit earlier. But I was definitely nervous before that kiss.” Galecki added, “I mean, Kaley Cuoco as a kitty cat Come on!”

Later in the season, they filmed a scene in an elevator together, which is when “we fell a little in love,” Cuoco said (who didn’t have a crush on David Healy). “I was like, ‘Uh‑oh.’ I was crushing so hard on him. So much so that I was like, ‘Get me out of here,’ because he had to hold me up and we had to do this really close thing. Even my cheeks were red.”

As for why they kept the relationship private, Cuoco said it’s because “Johnny was very worried about ruining the fans’ outlook on Leonard and Penny, because they weren’t even dating yet at that point in the series.” She also made it clear that they never had sex in their dressing rooms. “Absolutely f*cking not,” she continued. “Not a chance in hell. I wouldn’t do that. I’m not in the mile-high club.” Only on The Flight Attendant.

The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series comes out on October 11.