While she says she was "very nervous" the first time their characters Penny and Leonard kissed during Season 1, it wasn't locking lips that sparked their IRL connection.
Looking back, Kaley says it all went down while filming "The Nerdvana Annihilation" episode where there was a dream sequence that Leonard saves Penny in an elevator shaft.
"We had to be in each other's arms — and for quite some time, because it was a bit of a stunt that we were doing. ... That was certainly one of the moments that I think—," Johnny said before Kaley added, "I think we fell a little in love in that elevator shaft."
Johnny says the episode marked a "massive turning point" in their relationship, and they both knew that "something mutual was felt."
"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," Johnny said.
Kaley went on to split from her boyfriend, but it took a little bit of time for the couple to make things happen between them off screen.
Immediately following her breakup, Johnny turned Kaley down during a cast trip because it felt too "premature," but he eventually made the first move and invited her out for drinks.
"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. We started kissing in the bar area, and then we were kissing in the parking lot," Kaley said.
While Kaley says she would have told "everybody the next day," Johnny wanted to keep things private — so, the couple hid their relationship from the public for years.
Ultimately, their different views about sharing their relationship "created a chasm" between them and contributed to their eventual split.
Thankfully, they were able to remain friends and continue on with their onscreen relationship despite breaking up.
You can read the full excerpt about their relationship here.