Christopher Guest, co-writer of This is Spinal Tap (1984) and director/co-writer of Waiting for Guffman (1997), Best in Show (2000), A Mighty Wind (2003) and For Your Consideration (2006) has successfully created a niche for himself in comedic films that makes him a very interesting character study.
One of the finer points of Andrew Sarris' auteur theory is that the mark of a great director is being able to impose their mark within the confines of a studio system. In other words, a director being able to take material he doesn't like (that he usually gets stuck with) and make it work within his style. This is one of the most prevalent theories in film study and has generated a lot of heated debate over the years. Some of its detractors might point to someone like Vincente Minelli who took many projects he didn't like just so he could get the financing necessary to make his dream project "Lust for Life." Christopher Guest seems to support that notion because not many of his fans are familiar with the fact that Guest has directed more than four films. A one-time castmember on SNL in the 80's, he has directed some other films and TV movies, like "Dead on Arrival" and the Chris Farley-Matthew Perry vehicle "Almost Heroes," that have absolutely nothing in common with the five films that bear the "Christopher Guest and Co." trademark. The fact that he was willing to completely ditch his style of comedy to play by the studio's rules of conventional comedy before making movies his way, suggests that the studios didn't have a lot of faith in his brand of comedy. Again, I don't have DVD production notes from Almost Heroes with which to make that judgement. The question I want to ask, however, was that deserved?
His films feature a stock company of actors led by Christopher Guest as an actor, Fred Willard, Eugene Levy, Michael McKean, Harry Shearer (although he's been absent from a couple of them), Catherine O'Hara and Parkey Posey. Other actors such as Ed Begley Jr, Michael E Hitchkock, Bob Balaban, Christopher Moynihan, Jane Lynch, and Jennifer Coolidge have grown into the group as a secondary set of characters, although there are no clear lines.
The Christopher guest method of movie making is establishing the characters and then improvising most of their dialogue along the lines of what the characters might say. Improvizational comedy is not entirely new: Good comic actors have generally been known to ad-lib on the set if something better comes to their mind at the moment but the Guest films are based entirely around improv comedy and market themselves as such. It's also done in a very organic way that's key to the creative process.
It's actually kind of odd how this pattern is so pervasive. Many people within the Christopher Guest camp have not achieved widespread fame outside of it or if they do, it's usually for one or two roles that don't express their full ability. Other actors in the Christopher Guest stock company who are known by just one or two roles. Examples include:
Catherine O'Hara from Home Alone
Eugene Levy as the dad from American Pie
Bob Balaban as the studio boss from Seinfeld (although some might recognize him from Godsford Park)
Michael McKean from Lenny and Squiggy and a brief SNL stint
Jennifer Coolidge from the TV show Joey
One could say that Fred Willard and Parker Posey enjoy careers that enable them to capitalize on their strong spots. Willard seems to pop up fairly heavily in TV series and films and has developed a comic persona of an affable fake know-it-all who interferes with everyone around him. It was best perfected in Best in Show for which he might have been considered the highlight of that film. Posey is known for some independent films.
I'm sure if you had some objective way to rank every actor based on their ability to improvize comedically, the top slots would go to people not from the Christopher Guest camp: Jim Carrey, Vince Vaughn, Will Ferrell, Jack Black, Steve Martin, etc. Many Christopher Guest fans feel that Fred Willard, for example might be the funniest person on the face of the planet and would outperform all the others in an improv scene. It is hard to quantify exactly where a Fred Willard or Harry Shearer fits into the comic spectrum. I think it's very curious how the Christopher Guest stock company for all the praise they get (although, I have a feeling that took a negative turn with the lackluster film For Your Consideration), their stock company of actors has not moved up accordingly in fame. That Will Ferrell, Vince Vaughn and Jack Black still dominate comedy.
Perhaps, the Christopher Guest films are very organic processes and they take some underrated actors and combine them together to form the sum of their parts. I don't think a Fred Willard would be able to draw the star power that Ferrell, Vaughn or Black would. He doesn't have the ability to make you laugh like a stand-up comedian would, where he fires a couple of opening liners that have you howling. I think he wears down your guard over time. It's the small subtleties of his character's humor that become more and more exxagerated the longer you watch him. This worked very well in "Best in Show" and in "Waiting for Guffman" with him but I think that kind of character-based comedy that slowly builds up on you would be a hard sell on the studio level.