I remember Quentin Tarantino once talking about this in an interview with Charlie Rose (I think); the idea that there are some actors that can change the nature of a film simply because of who they are. That doesn't mean that the casting of an actor could change the ending of a film (although that has happened numerous times), but moreso that once an actor of a certain caliber is casted in a film, the film could be re-written in order to more properly fit that actor's talents.
In my eye, there are four basic kinds of actors:
the method actor - this could either be a really talented, famous actor or relatively notable actor who can succeed in being sucked into a role or completely transform himself in a role. Depending on how good the actor is, his transformation should make it so that when you watch him on screen, you identify him as the character in the movie rather than the actor playing the character. Notable recent actors of this ilk are Daniel Day-Lewis, Phillip Seymour Hoffman (formerly a character actor which I'll get to later), and Benicio del Toro. These actors more-or-less go all out every time they make a film and it should be no surprise that they have each one at least one Oscar. Some more classic examples are Marlon Brando, Robert Deniro (until the last decade), and Dustin Hoffman.
the character actor - a character actor tends be someone you've seen in lots of films but you don't recognize his name. he (or she) tends to be barely noticeable. they tend to take supporting roles in films although some have starred in films (Steve Buscemi and Gary Oldman, for example). Basically, a character actor is someone who is believable in every single role he's in. This actor is basically ubiquitous and very versatile. A character actor can more or less do anything you tell him to do and he can do it quite well. Some character actors specialize in a certain type of role (whether it's playing a cop or a criminal) or are typcasted in such a role (like Mark Strong, lately), but there is no denying the fact that a great character actor is a great person to have in your cast. Some examples are, as I've mentioned Steve Buscemi and Gary Oldman. Oldman can also qualify as a method actor, but aside from movie buffs, he's not really well known. He hasn't even been nominated for an Academy Award, which is absurd. But he is barely noticeable in most of the roles that he chooses, in a good way. I guess a classic example of the character actor would be like Peter Lorre from the '30s-'50s.
the classic actor - the "classic" actor is generally someone who is highly skilled in the theatrical arts, but his skills translates quite differently in film. I don't mean that in a bad way, of course, I simply mean that classic actors tend to be more verbose and expressive whereas method actors are more deliberate and... methodical. Obviously, classic actors were way more prevalent in the classic Hollywood era, people like Laurence Olivier. But there are still actors of that kind who exist today, I would consider most British actors to be classically trained actors. Even an American like Kevin Spacey who has done tons of theater work. Kevin Spacey, to me, is one of the few actors who can go from small and practical to big and boisterous. Another great example would be Nathan Lane or Bette Midler (I know I've not given any female examples up to this point, you'll have to forgive me). It's also important to note that method actors can be loud and abrasive too just in a more deliberate way.
last but not least, the celebrity actor...
The reason why I started this blog post was so I can talk about this fourth kind of actor. Now the celebrity actor or "the acting auteur" is a specific class of actor who may have come from any of the three classes of acting that I made up above. But this actor, at a certain point, has transcended all typical forms of classification. He is someone whose personality is so grand that often times, writers will write a film having those actors already in mind. It's hard to write a good film for a celebrity actors because you want to use the best of their talents and you don't want to misuse them. The one small flaw I found in The Thin Red Line is that George Clooney doesn't appear until the last half hour or so of the film. George Clooney, even in 1998, is so clearly a celebrity actor that him appearing in a small role like that just doesn't work so well. It's so clearly George Clooney no matter what role he's playing. Just like Jack Nicholson is obviously Jack Nicholson and Cary Grant was obviously Cary Grant. You write these movies with those actors in mind and you play to their strengths. The reason why I call them auteurs is because they have such a demanding presence both on set and in the film that everything is practically catered to them. They either have more control or an equal amount of control as the director on the set. The best directors know how to keep this kind of actor in line (like Aronofsky and Mickey Rourke in The Wrestler or Alexander Payne with Jack Nicholson in About Schmidt). There are bad directors though who can't handle actors of this magnitude (like with McG and Christian Bale... Bale is normally considered a method actor, but he could also be considered a celebrity actor especially considering how much Terminator Salvation felt like a star vehicle for him).
This is why you'll often see a actor of this nature work with the same director over and over again. Whether it's George Clooney and Steven Soderbergh, Leonardo Dicaprio and Scorsese, or Jack Nicholson with James L. Brooks. Celebrity actors like working with filmmakers that they're comfortable with. The more comfortable they are, the better they are at being themselves, or a version of themselves. What sometimes results is a film that's basically a showcase for that actor and may even depict a certain part of that actor's life (Warren Beatty in Shampoo, George Clooney in Up in the Air). The best types of films are ones that aren't hindered by the celebrity actor or is tailor made for a celebrity actor, but moreso it's a great film on its own that perfectly utilizes that actor's talents. It's a director who knows how to get the best out of that actor. Or maybe the actor is so enamored with the material that you get the best out of him. Either way, a celebrity actor at his best is usually a front runner for awards of all kind.
So, yes, Jack Nicholson may play a version of himself in every movie he's in and George Clooney may always be George Clooney... that's who they are, you can't change that. That doesn't mean they're incapable of starring in good movies or that they can't be believable with the characters they play. Great movies with celebrity actors are ones where the character and the actor are practically inseparable.