'For general biographical information, see Brian Baumgartner on Wikipedia.
Coming to The Office
Brian Baumgartner learned that The Office was auditioning, and he asked his agent to try to get an audition. His agent replied, "Well, they're looking for unknown people, but not like you unknown."
Brian Baumgartner originally auditioned for the role of Stanley, using his Kevin voice. After the audition, casting director Allison Jones told him there was another role they were looking to cast, and that's how he ended up auditioning for Kevin.
Brian Baumgartner was also the host of the podcast "An Oral History of The Office", in which he discusses the origins, struggles and successes of the show.