Kaley Cuoco, Johnny Galecki reflect on real-life romance during 'The Big Bang Theory'

Kaley Cuoco and Johnny Galecki's chemistry was so off the charts during "The Big Bang Theory" that they took their on-screen romance into the real world.

The pair respectively played Penny, the aspiring actress across the hall, and Leonard, the genius physicist, on the sitcom for 12 seasons from 2007 until 2019. While their characters ultimately ended up together after their on-and-off relationship, Cuoco and Galecki quietly dated early in the show's run.

"All the scenes up until we actually got together in real life, it was obvious there was always something there," Cuoco said in a book excerpt from "The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series" shared by Vanity Fair. "There was chemistry and we were crushing on each other. That was the whole first season until we actually got together for real."

The pair pointed to a scene they filmed together during that first season -- one where they are close together in an elevator -- as when the sparks became too real to deny.

"I think we fell a little in love in that elevator shaft," the "8 Simple Rules" alum said.

"We felt something, yeah. I think that was a massive turning point [in our relationship]," Galecki chimed in. "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."

The duo then quietly dated from 2008 to 2010.

Galecki described them as a "very loving couple" when they were together and said there was a point where they "did consider marrying" each other. As for why their relationship didn't pan out, he pointed to one of their biggest differences as being the reason.

"I think one of the things that created a chasm between us was my strict policies of privacy, and Kaley being very, very open about her life," the "Roseanne" alum said. "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," he continued.

Cuoco said she and Galecki have remained close friends throughout the years due to the bond they formed on the show.

"Johnny and I got so close through the show and our experiences. There was such a comfort level, and he always was so supportive of me," she said. "We would almost finish each other's sentences. We knew each other's characters so well, but he was just constantly a steadfast person in my life, on that show, on that set."