The Best Time Leonard Ever Broke Character On The Big Bang Theory

Playing an experimental physicist on "The Big Bang Theory" isn't technically rocket science, but that doesn't mean it's not a head-scratcher. Getting your mouth (let alone your mind) around the scientifically complex lines on the show is harder than the cast typically makes it look.

But there were occasions when even the actors who spent 12 seasons perfecting their "Big Bang Theory" line deliveries got tongue-twisted. It happened to Jim Parsons (Sheldon), it happened to Kaley Cuoco (Penny), and, yes, Johnny Galecki (Leonard) has also broken character and messed up a take.

The take may have been ruined for that episode, but fans still enjoy these moments. As much fun as it is to watch genius-level scientists wrangle with particle physics and the mechanics of human relationships, it's also nice to be reminded that the people playing them don't necessarily fully grasp those lofty concepts any more than we do.

However, Johnny Galecki can't put his funniest blooper down to the pressure of voicing an unpronounceable scientific concept in front of a live audience, TV audience, cameras, and castmates. His mess-up was more about love than science.

To appreciate Johnny Galecki's character break fully, here's a little refresher on Leonard and Penny's entire "Big Bang Theory" story. The slip happened in Episode 21 of the sixth season. At that point, on-off couple Penny and Leonard have weathered an unreturned "I love you," multiple breakups, reunions, jealousy, and a declined marriage proposal, and they're in a pretty good place. Penny has even managed to say "I love you" (albeit only 13 episodes previously.)

This scene comes at the end of an episode in which Penny has been having a minor crisis over not feeling passionate about anything the way Leonard is about physics, comics, and cosplay. Over dinner, she tells Leonard, "But then I realized I'm passionate about you," to which the live studio audience responded with a loud, collective "Aaaw." This was enough to make Galecki break character: He laughed and told them, "Oh, stop it." Cuoco also abandoned the scene for reality, with a jokey, open-mouthed "Did he just say that?" look at the audience. Galecki responded by reaching casually for his "wine" (no, it's not real alcohol), while shaking his head mock-scathingly.

It's not unusual for the cast of this particular show to flub dialogue about scientific theories. But seeing an actor moved out of character by the audience's reaction is a different kind of break. Audiences love Penny and Leonard as a couple, but apparently Galecki was over it that day.