Kevin Sussman Changed Stuart's Trajectory On The Big Bang Theory With One Improvised Line

A single improvised line by Kevin Sussman helped reshape Stuart Bloom's characterization on "The Big Bang Theory," according to the actor.

Stuart, who was first introduced in Season 2, Episode 20, "The Hofstadter Isotope," started out as a mild-mannered comic book store owner before gradually evolving into one of the show's most downtrodden supporting characters.

During an appearance at the 2021 Paris Manga & Sci-Fi Show, Sussman revealed a key moment in Season 3, Episode 7, "The Guitarist Amplification" — his third episode overall — that shaped his character's trajectory.

"There was a scene with Penny, and she comes to the comic book store and she asks me a question," he recalled. "And as she's leaving, I — which was not in the script — I muttered under my breath, 'I love you.' And from that point on, they sort of caught on to this whole Stuart vibe, where Stuart sort of regresses."

Sussman said the improvised line marked a turning point in how the writers approached the character, noting that Stuart "started out as a normal person" before becoming increasingly desperate as the series progressed.

He'll reprise his role, opposite fellow "Big Bang" alumni Lauren Lapkus (Denise), Brian Poesehn (Bert), and John Ross Bowie (Kripke) in the forthcoming spin-off "Stuart Fails to Save the Universe," coming to HBO Max in 2026.