Cast Full of Writers

A show or movie where the main cast also serve as the writers of the piece. Common in sketch comedies, as well as Video Review Shows and Internet Abridged Series due to their one-man-show nature. The trope can crop up in other types of show as well.

Film

 * Matt Damon & Ben Affleck, Good Will Hunting.
 * Many of the Judd Apatow-produced films and TV shows, and not just because his films are heavily improvised. Steve Carrell co-wrote The Forty Year Old Virgin and Jason Segel co-wrote Forgetting Sarah Marshall.
 * Casino Royale 1967 falls into this category - largely because the actual script was such a mess that producers had to get the cast (who included experience writers such as John Huston, Orson Welles and Woody Allen) to try and make some sense of it. Peter Sellers, the nominal star, also wrote a lot of his own material.
 * In Mad Magazine's parody of Animal House, when Dean Wormer threatens the Deltas, one remarks they have more power than he does because some of them are writers of the movie.

Live Action TV

 * Series creator Rob McElhenney, co-developer Glenn Howerton, and writer Charlie Day all have lead roles on Its Always Sunny in Philadelphia. Another co-writer, David Hornsby, plays recurring character Rickety Cricket.
 * Mystery Science Theater 3000 cast parts almost entirely out of the production staff, and primarily from the writers, to the extent that almost every bit part ever was played by someone who was employed to do something at Best Brains before they appeared on the show.
 * All of the replacement actors for the main characters also came from the crew, chiefly the writing staff. It was that kind of show.
 * Svengoolie writes pretty much all his own stuff.
 * Thirty Rock is a show about writers and producers, where Tina Fey plays her own writer/producer character while writing and producing the show. (Oh, she directs it too.)
 * A lot of Britcoms are created and written by the star or stars, especially ones who come from a stage or stand-up background. Examples include:
 * Fawlty Towers, written by John Cleese and his wife/ex Connie Booth
 * The Mighty Boosh is written by its stars, Julian Barratt and Noel Fielding, with additional material from Rich Fulcher.
 * Spaced was written by and starred Simon Pegg and Jessica Hynes (née Stevenson)
 * Ricky Gervais and Stephen Merchant wrote and starred in The Office and Extras.
 * The U.S. version of The Office also qualifies, as it includes three writers as cast members (Toby, Kelly, and Ryan), and two others have had smaller cameos. Steve Carrell has also written a few episodes.

Sketch Comedy

 * Monty Python's Flying Circus
 * The League of Gentlemen
 * Saturday Night Live, particularly in the first five years (1975-1980) due to the relatively small cast (and especially in the 1979-1980 season, when Lorne Michaels was looking for replacement actors for Dan Aykroyd and John Belushi, who left after season four), though the tradition of having SNL writers as either cast members or bit players continues to this day.
 * Second City, The Groundlings, and other Improv troupes generally.
 * The Kids in The Hall
 * SCTV
 * The State
 * The Goodies: two of the three main performers wrote it.
 * At Last The 1948 Show: four of the five main performers wrote it.
 * That Mitchell and Webb Look
 * Armstrong And Miller
 * Little Britain
 * The Catherine Tate Show
 * Almost Live
 * Les Luthiers
 * Hello Cheeky: three of the four main performers wrote it.

Video Review Shows

 * Pretty much anything on That Guy With the Glasses.

Western Animation

 * The Venture Bros has a lot of this - writer/creators Jackson Public and Doc Hammer spend a lot of time talking to themselves, between them they voice something like twelve characters.
 * Same thing with South Park, but more so. Trey Parker and Matt Stone voice pretty much anyone who's not a woman (though Trey Parker voiced Ms. Choksondik until she died in season six and also voiced a one-time female character -- the S'mores Schnapps lady -- on the episode "The Red Badge of Gayness." Mary Kay Bergman was supposed to voice her, but was replaced following her suicide). Or Chef (who was voiced by Isaac Hayes until season nine).