Sheldon (Jim Parsons) and Leonard's (Johnny Galecki) dysfunctional friendship is arguably the heart of CBS' "Big Bang Theory," but Galecki is the first to admit that the dynamic duo "could implode or split up at any second."

In a recent interview with Gold Derby, Galecki discussed the relationship between Leonard and Sheldon -- specifically why Leonard hasn't just slapped Sheldon for all his irritating behavior and constant putdowns. "You would be surprised -- or you might not be surprised -- how often I'm asked that question," Galecki laughed. "They really, truly need each other on some levels ... They have very different hearts, but they share the same brain in a lot of ways."

Galecki recalled an episode in the show's most recent season, "when Leonard talked about moving out and moving in with Penny, due to Sheldon ruining the 'Harry Potter' books for him. Of course, all of that discomfort or angst or frustration and exasperation that's been boiling under their dynamic for many, many years … of course it's going to come down to something as silly as ruining the end of the 'Harry Potter' books. It's all stuff that's been underneath for so long and it's a lot of fun to play -- it gives it a sense of unpredictability."

But, Galecki noted, "Leonard did slap Sheldon this season. You didn't see it, it was off-camera, but Leonard was laying in bed and Sheldon kept talking to him through the wall. It was Chuck Lorre's idea, during the live taping -- we had the studio audience there. And he said 'Johnny, could you just go out, leave the room, make a slap sound, and Jim'll holler.' I said, 'Really?' [Executive producer] Steve Molaro and I said 'Leonard's gonna slap Sheldon?' The audience went crazy, but it was a risky thing, I didn't think it would work, because I see how protective the audiences are of these characters. But it was like the Beatles walked in -- people went crazy when they heard that slap."

One "Big Bang" relationship that seems solid is Leonard's romance with Penny (Kaley Cuoco), despite Leonard's overseas job. Executive Producer Bill Prady told The Huffington Post that Leonard and Penny have "come to a good place for the moment, so I think maybe they'll get to be happy for a little while."

"Penny telling Leonard she loves him is pretty big. We didn't play it as a watershed moment. It was played as if she's always known this. It was touching," Galecki told The Hollywood Reporter in a recent interview.

"It took her more than five years to say it. That's why now you know with Penny that whatever she does is real. You know when she said it she truly meant it. That's why now Leonard is OK with going away and knowing that they're going to be OK," Cuoco agreed.

Watch the full Gold Derby interview with Galecki below:

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