I just pulled my copy of
Star Trek Magazine from Dec. 2001, the TMP issue. It features an interview with Collins about the movie. The third paragraph starts: "In 1978, Stephen probably wouldn't have been interested in taking a regular role in a TV series. He was gathering a lot of attention and everyone agreed he was destined for a career in the movies."
Collins heard about the part from his agent and was convinced to go in to audition. Collins wasn't a
Star Trek fan and wasn't particularly interested, but his agent convinced him that a part in the
Trek movie couldn't possibly hurt him. If it was a hit, then he was in a hit movie and boosted his profile. If it flopped, no one would blame him. Collins also said that he thought the fact that he wasn't a
Trek fan worked in his favor, because he wasn't as nervous as some of the other actors auditioning for the part. I'm sure the prospect of working with Robert Wise was also appealing.
Collins did do some TV jobs before
Seventh Heaven, though: He heard about TMP while he was guest starring on
Charlie's Angels.