Kaley Cuoco & Johnny Galecki were dating other people. Then a scene in a lift changed everything.

Kaley Cuoco and Johnny Galecki are reminiscing about the time they played on-screen love interests while also dating in real life.

For 12 years, the actors played Penny and Leonard on the wildly popular sitcom, The Big Bang Theory.

Their characters are neighbours: Cuoco, the blonde, aspiring actress who lives across the hall, and Galecki, the socially awkward experimental physicist who has a crush on her from the moment they meet.

Image: Warner Bros.

It was an unconventional pairing that just worked, on and off-screen.

Cuoco, now 36, and Galecki, now 47, dated exclusively for two years from 2008 to 2010. But Cuoco says she had a crush on her co-star way before that.

"I had a very big crush on Johnny early on," The Flight Attendant actress said in excerpts from Jessica Radloff's book The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series obtained by Vanity Fair.

"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 continued. 

"Then, when I found out he had eyes for me too, I was like, 'Uh‑oh, this is going to be trouble'."

Much like his character on the show, Galecki admitted he was oblivious to her feelings towards him.

"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."

"All the scenes up until we actually got together in real life, it was obvious there was always something there," Cuoco explained. "There was chemistry, and we were crushing on each other. That was the whole first season until we actually got together for real."

The actress added that the first kiss the pair had on the show as Penny and Leonard was also technically the first time Cuoco and Galecki kissed too.

"I was very nervous with both [our on-screen and real-life first kisses]," she said. 

"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."

Penny and Leonard's first kiss. Image: Warner Bros. It was during season one episode 14, during a dream sequence when Leonard saves Penny in an elevator, that the actors decided it was time to give into their feelings for each other.

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".

"We felt something, yeah," Galecki agreed. 

"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 recognise it and surrender to it."

While Cuoco wanted to announce they were dating after their first date, Galecki wanted to keep it quiet – so they did.

The actress added they always kept it professional: "We never even had [sex] in our dressing rooms," she said.

"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."

And although the actors only dated for two years, they are still great friends; and think the success of their characters was all to do with just how well they got along when the cameras weren't rolling.

"I think a lot of what made Leonard and Penny work so well was my relationship with Johnny off camera, which turned into such sarcastic banter that bled into Penny and Leonard," Cuoco explained.

"Johnny and I’s relationship, in a way, was mimicking Penny and Leonard. They were always giving each other s**t, and Johnny and I have a similar relationship – which you do after years of being together," she added.

"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."

Feature image: Warner Bros/Getty.