There is a certain caliber of actor for whom acting is not required: All they need to do is be themselves, the intensity of their "self" either dialed up or dialed down depending on the movie. Think of it, if you can without inducing nausea, as the Larry The Cable Guy School Of Acting. Much in the same way that Larry The Cable Guy always plays Larry The Cable Guy, Jack Nicholson always plays Jack Nicholson, Robin Williams always plays Robin Williams, Dane Cook always plays Dane Cook, and Jennifer Aniston always plays Jennifer Aniston (except for that one movie where she had a terrible Southern accent, then she was Jennifer Aniston with a terrible Southern accent)–only the words and the wardrobe change.
In some ways, being part of the Larry The Cable Guy School Of Acting is a sign of success. Your public persona or comedic persona or trademark movie persona is so well known that you've become typecast as yourself. But mostly being a member of the Larry The Cable Guy School Of Acting, is a guarantee that eventually people will get tired of seeing you be so, you know, you-like.
Unless, of course, you're Diane Keaton, who has been steadily playing neurotic, stammering, whiny-yelling, clumsy, goofy Diane Keaton since at least 1991's Father Of The Bride to (as of right now) 2008's slapsticky Smother.
In truth, Diane Keaton has been playing Diane Keaton since Annie Hall, but through the years her portrayal of herself has lost almost all of its warmth and genuine feeling, and just gradually become a broad, flat Diane Keaton who is subject to many, many hilarious pratfalls. Diane Keaton's Diane Keaton performances just get more and more depressing with each terrible family comedy she does. Pretty soon she'll only speak in whiny stammers, and walk by constantly falling down.
Dax Shepard, on the other hand, does an excellent (read: hopelessly irritating) Dax Shepard.