I love threads like these. People name, usually good actors, and give reasons like "Terrible in everything!" without elaborating. For example people who hate Maguire. Have you seen The Ice Storm, Wonder Boys, Ride with the Devil and/or Brothers? Have Knightley haters seen Pride & Prejudice and Atonement?
I mean Will Ferrell does play the same character a lot, but I doubt the doubters have seen Melinda/Melinda, The Producers and Stranger than Fiction. For the record, I hate the criticism that mainstream comedians "always play the same role." All comedians create or attempt to create a signature personality that audiences like and therefore audiences will pay to see that personality on the screen in funny situations. Yes, Ferrell always plays the same character. Adam Sandler always plays the same character. Ben Stiller always plays the same character. So does Owen Wilson, Vince Vaughn, Jack Black, Steve Carrell, Seth Rogen, Jonah Hill, Micahel Cera, Dan Akryod, Eddie Murphy, Bill Murray, Steve Martin, Chevy Chase, Martin Short, Chris Farley, John Belllucci, Abbot and Costello, Groucho Marx, Bob Hope, Doris Day, Richard Pryor, Mel Brooks and The Three Stooges.
There are exceptions to this. Murray, Wilson and Stiller can do work like Wes Anderson films and Lost in Translation, Steve Martin can do Shopgirl...but let's be honest comedians all find their niche and use it. So one should say they do not like that comedian's style than the criticism "They always play the same character." For example, I hate the character Jim Carrey plays. I don't think he is funny, but annoying and overcompensating. While I can still say Carrey is a good actor when he stars in Man on the Moon or Eternal Sunshine and admit he is a good actor--I still hate his Ace Ventura/Mask/Dumb and Dumber/Liar Liar/Yes Man character.
As for the rest of the thread. Minus extreme examples (Paul Walker, Miley Cyrus, Madonna, etc.) I am always open to any actor if they look to reinvent themselves or find a good role in a good film.