He joins the team only by the middle of the film, and for the rest of it he's all like "I just work for these guys, I'm not crazy like them", doesn't even have many lines. The Wikipedia article for the film even has him as a secondary cast member or a guest-actor rather than one of the leads.

Was this because he wasn't as well known an actor at the time as some of the others (I'm not American, I don't know how successful the actors were prior to Ghostbusters)? Did he have conflicting schedules, and couldn't dedicate fully to this film? Or was it a racial issue, since he's the only black character in the film? (not being racist, just wondering if it could have been an reason)