Cuoco, 36, and Galecki, 47, starred as onscreen love interests Penny and Leonard Hofstadter on CBS's hit sitcom The Big Bang Theory from 2007 to 2019. Along the way, their chemistry found its way off screen as the pair dated in real life from 2008 to 2010.
The exes opened up about the start of their romance in an excerpt from an upcoming oral history of the show, with Cuoco confessing, "I had a very big crush on Johnny early on."
"I was so not even hiding it," she said in an exclusive excerpt from The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series (written by Jessica Radloff) shared with Vanity Fair. "He has such swagger. We were both dating people at the time, but I only had eyes for Johnny. Then, when I found out he had eyes for me too, I was like, Uh‑oh, this is going to be trouble."
Still, Galecki was oblivious to Cuoco's feelings for him at first.
"I have a healthy ego, but you really have to kind of club me over the head to let me know you're flirting with me," he said. "I had zero idea of any sort of crush before Kaley and I began dating."
Alberto E. Rodriguez/Getty; Jason LaVeris/FilmMagic
But according to Cuoco, there was no denying their characters' "chemistry," even before the pair made their attraction known off set.
"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."
Michael Yarish/CBS Leonard (Johnny Galecki) and Penny (Kaley Cuoco) on The Big Bang Theory
Speaking about Penny's first kiss with Leonard, which also served as the actors' first kiss, Cuoco referenced a sexy kitten costume Penny wore to Leonard's Halloween party.
"I knew I looked real cute in that outfit, so I was feeling really good about myself," the Meet Cute star said with a laugh. "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 chimed in, "I mean, Kaley Cuoco as a kitty cat? Come on!"
That said, Cuoco did admit, "I was very nervous with both [our onscreen and real-life first kisses]. I mean, I was kissing him as Penny before we ever dated, and it's weird when you have a crush on someone and you're kissing them as actors."
Robert Voets/CBS via Getty Cuoco and Galecki on The Big Bang Theory
As for when the actors decided to finally give in to their feelings for each other, they pointed to the 14th episode of season 1, "The Nerdvana Annihilation," during a dream sequence when Leonard saves Penny in an elevator shaft.
Though the physical filming of the scene was "not an enjoyable experience," according to Cuoco, she does "think we fell a little in love in that elevator shaft."
Galecki agreed, "We felt something, yeah. I think that was a massive turning point [in our relationship]. At that point, both she and I knew that something mutual was felt, and that it was going to be more of a distraction from the work to try and continue to ignore it than to actually recognize it and surrender to it."
Over a period of time, the couple's connection grew, until Galecki invited Cuoco for a drink on the spur of the moment.
"I was so excited when he texted me. I was like, Oh my God. And I knew that was going to be trouble — getting a glass of wine with him. I knew. And we kissed at the bar! It's so dark in there that you almost feel like you can do anything, but then it was like, Uh‑oh, I think this is really going to be bad," she said with a laugh.
"We started kissing in the bar area, and then we were kissing in the parking lot," she continued. "Johnny can get really in his head and want to hide, whereas I'm the direct opposite. I would have told everybody the next day at the table read. I would have announced it and said, We're together! But he was like, 'We can't tell anyone!' So we kept it quiet."
Or at least they thought they did — though they did cop to getting caught by Big Bang creator Chuck Lorre at least once. Recalled Galecki, "One time Chuck busted us hiding behind a car in the parking lot. It was after a show ... and we ducked down, like kids hiding from their parents! And he totally busted us, but he was classy about it and didn't say anything. We just didn't want anybody to worry."
For his part, Lorre said, "I can't remember that. That might be one of those things I compartmentalized like, I can't deal with this, I don't have the mental, emotional bandwidth to deal with an on‑set romance."
Cuoco deemed the response "such a Chuck answer!" She insisted she and Galecki kept it professional, though, and "never even had [sex] in our dressing rooms. Absolutely f---ing not. Not a chance in hell. I wouldn't do that. I'm not in the mile-high club. None of that…."
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Looking back, the friends–turned–exes–turned–friends–again are happy with how their off-screen dynamic positively impacted their onscreen appeal.
"I think a lot of what made Leonard and Penny work so well was my relationship with Johnny off camera," said Cuoco. "They were always giving each other s---, and Johnny and I have a similar relationship — which you do after years of being together. We've always had each other's backs, and you can't lose that. And I do think it happened on-screen. It made our relationship funny and endearing."