A player can be disappointed with their role and still contribute honestly within it.

Disappointment does not have to show up in the work. Someone who wanted a bigger part can sulk through a smaller one, or do it properly while still wishing it were different, and the second is neither dishonesty nor cheerful acceptance. Two things are simply true at once, and both get to stay.

If you are in a role you did not want, decide what doing it well actually looks like and hold to that. Keep the wanting for the people who can do something about it.