Some actors we have seen pop up as side characters in movies were actually huge stars in their day. They used to headline films, starring as the main actor in the film with their name highlighted as the star attraction on the movie poster until they just became side actors for many possible reasons - age, not big box office money makers, hard to work with, choosing to act less, etc.
It makes one appreciate an actor like Clint Eastwood who has headlined practically all his films for 50+ years.
Mia Farrow: Reckless (1995)
Kirk Douglas: Tough Guys (1986)
Christian Slater: Hard Rain (1998)
Roy Scheider: The Fourth War (1990)
Alec Baldwin: Mercury Rising (1998)
Dan Aykroyd: Blues Brothers 2000 (1998)
Can anyone list more?