Possible Clues:

• Cast in similar roles
• The sort to be thrown, given similar parts?
• Given only similar roles
• Not having many different parts?
• Eg Roman set always chosen to play villains, say?
• Kind and shy, in part pigeonholed
• Acts badly after work at keyboard getting the same parts?
• Stereotype
• In extremes of torment, Spacey furious, given predictable roles
• Unable to broaden one's roles