Over the course of its 12 seasons, the CBS sitcom developed a deep roster of supporting characters to go alongside the main cast, which started out as five characters before adding two more in subsequent seasons. Notable recurring characters on the show include Sheldon's (Jim Parsons) mother, Mary Cooper (Laurie Metcalf), and Leonard's (Johnny Galecki) mother, Beverly Hofstadter (Christine Baranski), as well as Stephen Hawking and .

One of the series' biggest recurring characters, however, is Stuart Bloom. Played by , Stuart is the owner of the comic book store most often frequented by Sheldon, Leonard, Howard (Simon Helberg), and Raj (Kunal Nayyar). Stuart ends up befriending the group and is often depicted as depressed and unlucky in love. He dates both Penny (Kaley Cuoco) and Amy (Mayim Bialik) over the show's run, but of course, neither relationship lasts. And Sussman revealed that an unscripted line he added in relation to his brief affair with Penny permanently changed his role on the show. Here's what he had to say.

Sussman's improvised line made the character depressed and unlucky

At a panel at the 2021 Paris Manga & Sci-Fi Show (via ), Kevin Sussman spoke about his decision that Stuart should whisper "I love you" as Penny walks away in Season 3, Episode 7, "The Guitarist Amplification," explaining that this improvised line changed the course of the whole character. Prior to this, Stuart had been written as an average person, but depression ended up becoming his defining trait, setting him apart from the show's other recurring characters.

Season 12 sees Stuart finally find love with , a relationship that Sussman would've liked to explore further. Unfortunately, "The Big Bang Theory" came to an end after this season, so audiences didn't get to see much of the couple. Sussman believes that the writers were unaware that Season 12 was going to be the show's last until it was too late and concluded that he wouldn't have been averse to "The Big Bang Theory" continuing.

Sussman almost played another major character

It turns out, Kevin Sussman's role as Stuart was the culmination of several attempts by co-creators Chuck Lorre and Bill Prady to get him on board. Sussman was actually their first choice to play main character Howard Wolowitz, with the part even having been written for him. Unfortunately, as Prady noted in  ABC wouldn't release Sussman from his existing contract for the show "Ugly Betty," forcing CBS to drop Sussman and ultimately cast Simon Helberg as Howard instead.

As he shared at the 2021 Paris Manga & Sci-Fi Show (via ), Sussman was eventually approached by the team once again, this time for the role of Barry Kripke, who first appears in Season 2, Episode 12. Scheduling conflicts with a movie Sussman was working on meant this became the second role Sussman had to walk away from, with John Ross Bowie being cast instead.

Eventually, Sussman was cast as Stuart, who was initially intended to just be a one-episode character. However, the team decided to bring him back a few more times, and in part thanks to Sussman's improvised line, he ultimately appeared in a total of 84 episodes over the show's run.