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The 'Big Bang Theory Cast' never went off script except for one character— and it's not who'd think

'The Big Bang Theory' was a show that was truly dependent on its heavily-written scripts and quick-witted dialogue, barely allowing for much else.

The Big Bang Theory was notorious for having writers who didn't take chances with the script and hence improvisation rarely made it onto the air. When jokes fell flat, the writing team would immediately get together, penning new lines rather than letting actors ad-lib their way through scenes. Given that the show tightly controls every dialogue, one spontaneous moment that slipped through the cracks is even more remarkable today.

Kevin Sussman, who played comic book store owner Stuart Bloom, was privy to this exact writing process when he joined the show in its second season. As he explained in 2019, any time a joke needed to be swapped out, the writers themselves would rewrite rather than rely on any kind of improvisation. It was Sussman's own ad-libbed addition, however, that would turn his character from a well-adjusted businessman who briefly dated Penny into an insecure but hilarious clingy friend. Sussman's quick thinking in one scene forever changed his character's journey making it one of the show's most popular supporting roles. "I didn't realize at the time that it was going to be more than one episode," Sussman admitted once, as per Cheat Sheet. "I thought it was just going to be a one-shot guest star type of thing, and then it turned into basically an entire career."

The turning point came in the Season 3 episode The Guitarist Amplification. During a scene when Penny came to the comic book store, Sussman added an unscripted line. "She comes into the comic book store and she asks me a question. And as she's leaving, which was not in the script, I muttered under my breath, 'I love you,'" Sussman remembered during a Q&A at Roster Con. The ad-lib caught the writers' attention and influenced Stuart's future portrayal.

With just that one line, Stuart went from a cool, calm businessman to a lovable, awkward figure— one of the best arcs in the show. "It was the first time really, that Stuart went from just being like a normal guy to being desperate," Sussman explained. The nuance paid off, with co-star Mayim Bialik later labeling him her favorite supporting actor. "He does a lot with a little and has an understated way of handling things that otherwise could be too much," she told The Things.

Sussman's portrayal of Stuart was most likely informed by his own experience working at the New York City comic book store named Jim Hanley's Universe, during his time at acting school. He told TV Store Online, "I wasn't a fan of comics before I started working there but I became a fan afterward." By the series finale, Stuart had become a core character, and showrunner Steve Molaro made sure he was part of the last episode. "We knew we wanted Kevin Sussman to be part of the finale because he's such a part of the show," Molaro said. Stuart's journey came full circle as he finally found both professional success and personal happiness.

Although The Big Bang Theory had some of the best storylines, Sussman's spontaneous response resulted in one of the most likable and relatable characters changing for viewers. His contribution proved that sometimes in television, the most authentic moments come from those rare occasions when actors trust their instincts and take creative risks.