7. Stuart Tells Penny, "I Love You"
Kevin Sussman ended up appearing in more than 80 episodes of the show as long-suffering comic book store owner Stuart Bloom.
Stuart first appeared in the late season two episode "The Hofstadter Isotope," and ends up dating Penny (Caley Kuoco) for a brief spell until he realises that she and Leonard have unfinished business to say the least.
Sussman never expected the part to last more than a single episode, but ultimately earned himself a lucrative recurring role after ad-libbing a memorable line in the season three episode "The Guitarist Amplification."
After Penny enters the comic book store, he quietly says "I love you" while she walks away, underlining that he still desperately carries a torch for her.
The declaration was entirely unscripted and improvised by Sussman on the spot, and in his own words helped kickstart a series-long arc where Stuart becomes increasingly troubled:
"Usually there is very little ad-lib or improv on that show, but the writers laughed a lot and, after that moment, they started having me back and I noticed that they started having the character of Stuart sort of regress... It was the first time, really, that Stuart went from just being like a normal guy to being desperate.”
While supporting cast members certainly shouldn't be going into business for themselves on the regular, if you pick and choose your moments carefully like Sussman clearly did, it can change your life.