Actors / actresses that never change hairstyles even across different movies

Roy Scheider

Gary Coleman

Goldie Hawn
Reese Witherspoon
Diane Keaton

Cary Grant

Telly Savalas

Funny you mention Jennifer Aniston. Remember when she came back from summer break while on Friends, and she had changed her hair from "the Rachel"? Fans went nuts screaming they wanted the old look back. The next year her contract was rewritten: In order to maintain the character's look, she would have to get approval if she wanted to do it again.

I think the same thing happened to Kaley Cuoco after she got that horrible bob cut between seasons.

Bruce Willis had 10 different hairstyles in The Jackal ,let alone in career.

Jason Statham had different hairstyles in early movies. Revolver,Snatch,..

Al Pacino?? Godfather,Scarface,Dog Day Afternoon all different.

I agree with Roy Scheider,Meg Ryan,Cary Grant.

I would add:
Paul Newman , Ben Affleck , Liam Neeson(with few exceptions,where he only Cut little bit hair,but same hairstyles) , Marylin Monroe , Ricardo Darin , Whoopi Goldberg , Lucy Liu.

There are actors who are often with one hairstyles,but occasionally changed like Jamie Lee Curtis,Sandra Bullock,Ryan Gosling.