Kaley Cuoco on Falling in Love with Johnny Galecki on ‘The Big Bang Theory’

“The Big Bang Theory” alums Kaley Cuoco and Johnny Galecki are getting candid about falling in love on set.

Vanity Fair shared an excerpt from the new oral history “The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series,” in which the stars revealed the night they became more than friends.

The stars played love interests Penny and Leonard on the sitcom from 2007-2019, and dated in real life from 2008-2010.

In the excerpt, Kaley confessed, "I had a very big crush on Johnny early on."

"I was so not even hiding it," she insisted. “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.’"

Galecki, however, was clueless.

"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."

Cuoco pointed out, "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."

The exes actually shared their first kiss in character the night Penny wore a sexy kitten costume to Leonard’s Halloween party.

"I knew I looked real cute in that outfit, so I was feeling really good about myself," Kaley said. "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!"

Cuoco said she was nervous about their first kisses on-screen and off, explaining, “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."

Looking back, they both agreed that their connection grew while filming “The Nerdvana Annihilation,” which was the 14th episode of Season 1.

The episode involves a dream sequence in which Leonard saves Penny in an elevator shaft.

Kaley said they “fell a little in love in that elevator shaft,” Johnny 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."

At some point following the episode, Galecki asked Cuoco out for a drink.

"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.’”

The actress revealed, "We started kissing in the bar area, and then we were kissing in the parking lot. 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."

Galecki said that despite trying to keep the romance under wraps, show creator Chuck Lorre did catch them kissing.

“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!” Johnny said. “And he totally busted us, but he was classy about it and didn't say anything. We just didn't want anybody to worry.”

Chuck insisted he didn’t recall the incident, saying, "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.’"

Kaley called that “such a Chuck answer!” adding that they kept it professional and “never even had [sex] in our dressing rooms. Absolutely f**king not. Not a chance in hell. I wouldn't do that. I'm not in the mile-high club. None of that…."

Cuoco also reflected on how their off-screen relationship influenced their on-screen one.

"I think a lot of what made Leonard and Penny work so well was my relationship with Johnny off camera," she said. "They were always giving each other sh*t, 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."

These days, Kaley is now dating actor Tom Pelphrey and she recently told "Extra" it was love at first sight.

“When I met Tom, it was absolutely love at first sight," she said, revealing, “We have the same management team. My manager actually took me as her guest to the ‘Ozark’ premiere, and I met him there. It was like the angels started singing. I was like, ‘Hallelujah!’ It was very magical… it was perfect.”