The pair played couple Penny and Leonard on the CBS series until its conclusion in 2019.
They struck up a close friendship on the show’s set, and started a two-year relationship ahead of filming season two in 2007.
Speaking in an excerpt fromThe Big Bang Theory: The Definitive, Inside Story, which has been obtained by Vanity Fair, Cuoco remembers having a “very big crush” on Galecki, whom she said had “such swagger” on set.
The pair had their first on-screen kiss during a Halloween-themed episode titled “The Middle Earth Paradigm”, for which a “nervous” Cuoco was dressed as a cat.
She said: “I knew I looked real cute in that outfit, so I was feeling really good about myself. 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.
“All the scenes up until we actually got together in real life, it was obvious there was always something there. There was chemistry and we were crushing on each other. That was the whole first season until we actually got together for real.”
Galecki described the shooting of a dream sequence for an episode titled “The Nerdvana Annihilation” as a “turning point”. After rehearsals one day, they ended up “getting a glass of wine” together at a restaurant, which Cuoco said she “knew was going to be trouble”.
“We kissed at the bar!” she said, adding: “It’s so dark in there that you almost feel like you can do anything, but then it was like, “Uhâoh, I think this is really going to be bad. We started kissing in the bar area, and then we were kissing in the parking lot.’
Despite being together for two years, ending their relationship in 2009, they decided to keep their romance a secret – but they did tell their co-star Jim Parsons, who played Sheldon Cooper.
Galecki said that the decision to keep the relationship quiet mostly stemmed from him, which he suggested is what contributed to the break-up.
“It was a lot of fun to teach one another different things, but then when I came to wanting different things, that made things more complicated. Kaley being very, very open about her life. I was very uncomfortable with being public about it, and I think that hurt Kaley’s feelings a little bit, and I can understand that.
“It certainly wasn’t because I was embarrassed of her or our relationship, but I wanted to be protective of the audience’s acceptance of Penny and Leonard, and without distraction from the tabloids. I’m private in general.’
Cuoco said they ‘ran out of things to talk about” due to spending so much time together professionally as well as personally.
In the same book, show’s creator Chuck Lorre denied adding more sex scenes for the actors fter they broke up
The Big Bang Theory: The Definitive, Inside Story will be published on 11 October.