Clint Eastwood was in "Rawhide"
I don't think that qualifies as a "bit" role, since he was the star of that show, which ran for about 8 years.
The first thing that came to mind was Eastwood's role in "Revenge of the Creature". I did not see that movie until I had already seen his spaghetti westerns, so it was more like why is a famous actor appearing in a bit part like that?
I have been looking at Roger Corman's filmography at IMDB. A lot of actors and directors who became famous have been involved in his movies.