Kevin Sussman was supposed to play Howard on The Big Bang Theory, but he ended up missing the role and playing Stuart instead. Debuting in The Big Bang Theory season 2 episode 20, "The Hofstadter Isotope," Stuart was the owner of the comic book store that the guys hung out in. He went on to become the unofficial eighth member of the Pasadena gang and arguably the show's most notable ing character. Had things gone differently, however, Sussman could have played Howard Joel Wolowitz.
As revealed in Jessica Radloff's book, The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series, the role of Wolowitz was actually written for Sussman. The whole casting process was already finished and Sussman was already celebrating with his manager when he received the call that his new job wasn't pushing through anymore, because he was previously optioned by ABC for Ugly Betty and the network wouldn't release him. Read The Big Bang Theory co-creator Bill Prady and casting director Ken Miller's recollection of what happened below:
Prady: The part of Wolowitz was written for Kevin. We loved him, especially after he first came in and auditioned for Leonard in the first pilot. [ABC] wouldn’t let Kevin out of his contract, even though his arc on Ugly Betty was done at the time.
Miller: ABC had an option on Kevin and wouldn’t release him. And then the sad part is, he was never on Ugly Betty after that again. They kept him from getting that role on Big Bang.
What Kevin Sussman’s Howard Could’ve Been Like
In the same book, Sussman recognized how fascinating it was that he was supposed to play Howard, who was confident and upfront, and ended up portraying Stuart, who was the opposite. As seen on The Big Bang Theory, the earlier version of Howard was sleazy and even offensive at times. Stuart, on the other hand, had always been meek and awkward, especially around women. Neither Prady nor any of The Big Bang Theory executives have talked about what Sussman's screen test for Howard was like, so, it's difficult to imagine what his version of the character would have been. But the fact that they fleshed out the role with him in mind means that he was good in the part.
Before Radloff started to work on the book, Helberg had no idea that his role as Howard was supposed to be for Sussman. As it turns out, the Stuart actor never mentioned it throughout his years on The Big Bang Theory, because he didn't want to burden Helberg with it. Perhaps that was the best call, considering the situation, as getting it out in the open could have changed the dynamic between the actors, especially since Stuart became very close to the Wolowitzes.
What makes this more interesting is that Howard wasn't the only main The Big Bang Theory character that Sussman almost played. As revealed by John Ross Bowie, he auditioned to play Leonard before he nabbed the role as Caltech's Barry Kripke. Around the same time, CBS was also doing auditions for Sheldon before Jim Parsons was even in the picture. Sussman tried for that main role, as well, but didn't get it.
Source: The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series