Written by Cast Member

Sometimes actors want to have a say in the words they're given; sometimes they want to stretch their artistic muscles; sometimes they look at the scripts they have to do and think, "I could do better than this!" When this happens, and the producers are on their side (or they've been told to be on the actors' side), you have something written by a cast member.

Much less prevalent than Directed by Cast Member (writing is less glamorous than directing, for a start, and a bit harder because of having to actually set down a whole story instead of bringing something else to the screen), particularly in these days of arc-driven television shows.

Compare Directed by Cast Member, obviously, as well as Cast Full of Writers for when the vast majority of the writing staff is also up on screen.

Examples of Written by Cast Member include:

Film -- Live-Action

Sylvester Stallone has written or co-written a lot of his movies, even the ones that he didn't pen from scratch (like Cliff Hanger and The Expendables).
Italian movie star David Warbeck (that was his real name; he was from New Zealand) wrote some of his movies like Treasure Of The Golden Cobra, although not with credit.
Simon Pegg co-wrote and starred in Shaun of the Dead and Hot Fuzz.
- Pegg also contributed a few lines to Run Fatboy Run.
Eddie Murphy wrote the screenplay for Harlem Nights, as well as directing it.
Steve Martin wrote the screenplay for Bowfinger (and in fact he wrote or co-wrote many of his films).

Live-Action TV

Small Wonder had a few episodes written, at least in part, by Dick Christie.
Since the 1980s, Sonia Manzano has been both a performer and a writer on Sesame Street.
During their days on Head of the Class, Brian Robbins and Dan Schneider had their first writing credits on the "Will The Real Arvid Engen Please Stand Up?" episode. And the rest is history.
Although "Saving Private Leo" is one of the few episodes of Charmed to specifically be about the Halliwells' favourite Whitelighter, and Brian Krause (Leo) is the only cast member of Charmed to co-write an episode, this isn't it - he has co-story credit on "Sense And Sense Ability." He's said the finished product was different from what he turned in.
Unlike Directed by Cast Member, very few episodes of Star Trek (any incarnation) qualify - in fact, the only episodes of the entire run to have involved cast members are "The Muse" on Star Trek: Deep Space Nine(co-written by Majel Barrett-Roddenberry) and "Life Line" from Star Trek: Voyager (co-written by Robert Picardo). Walter Koenig wrote "The Infinite Vulcan" for the animated series, but that doesn't count because Chekov wasn't on the cartoon.
Barry Watson wrote an episode of 7th Heaven (not one of the two he directed).
Robert Culp loved doing this on his series - Trackdown, I Spy and The Greatest American Hero all had episodes he wrote (AND directed).
John Schneider co-wrote and directed "Opening Night At The Boar's Nest," the Series Finale of The Dukes of Hazzard.
In addition to being the only person to appear in every episode of M*A*S*H, Alan Alda also wrote numerous episodes (and was one of the writers of the Series Finale to boot).
Both Dean Cain and Teri Hatcher wrote episodes of Lois And Clark: The New Adventures Of Superman.
Tina Fey has written or co-written a number of Thirty Rock episodes, including the pilot. This is more a case of Descended Creator, however, as she did not originally plan to act on the show.
Jack Klugman, who made no secret of his views on the standard of writing in TV, wrote or co-wrote four episodes of Quincy in addition to having showrunner Glen A. Larson thrown off the show and eventually getting writers more to his liking. Including his own brother and sister.
Steven Seagal's TV series True Justice ("Created by Steven Seagal") featured scripts co-written by the great man himself.
Don Adams co-wrote two episodes of Get Smart.
Peter Falk wrote one episode of Columbo.
Roger Smith wrote several episodes of 77 Sunset Strip.
Done quite a bit on The X-Files, especially in later seasons. David Duchovny wrote (and directed) two episodes: "The Unnatural" and "Hollywood AD", while co-developing storylines for seven others. Gillian Anderson wrote (and directed) season seven's "all things." William B. Davis (who plays CGB Spender) wrote "En Ami."
Seth Rogen has a writing credit on a few episodes of Undeclared.
Two episodes of Wizards of Waverly Place were written by David Henrie.
Steve Smith, who portrayed the title character of The Red Green Show, also co-wrote every episode. He often worked with Rick Green (who played Bill), and in later seasons, Peter Wildman (Buzz Sherwood) and Bob Bainborough (Dalton) contributed as well.
Two episodes of Farscape (Season 3's "Green-Eyed Monster" and Season 4's "John Quixote") were written by Ben Browder.
Christopher Judge (Teal'c) wrote multiple episodes of Stargate SG-1.
Both Amy Poehler and Nick Offerman have written episodes of Parks and Recreation.
Michael Imperioli wrote several episodes of The Sopranos.
Michael Landon penned several episodes of Bonanza, wrote more episodes of Little House On the Prairie than anyone else and was a regular writer for Highway to Heaven (the only one of his three series which he also created).

Radio

In 1955 The Archers had an episode where Grace Archer, the wife of Phil, was killed off. (It may or may not have been coincidence that the episode went out on BBC radio the very night commercial television began.) The script had Grace's fate be conveyed in the final line of dialogue thusly: "She... she died in my arms... on the way to hospital." However, Norman Painting (who played Phil) suggested the line go "In my arms... on the way to hospital... she's dead!" Suffice to say that not only was this one of the most talked-about episodes of the soap, but Painting went on to write many, many episodes as well as act in them.
Radio Active had a Cast Full of Writers (so full, in fact, that Helen Atkinson Wood was the only one who didn't chip in on that front).

Western Animation

Dan Castellaneta wrote the odd episode of The Simpsons with his partner Deb Lacusta (he also co-wrote at least one sketch of The Tracey Ullman Show).
Alex Bornstein (the voice of Lois Griffin) has been a staff writer on Family Guy since the beginning. She even wrote part of Stewie Griffin: The Untold Story.
The majority of the voices on The Venture Bros. are done by Jackson Publick and Doc Hammer—who also happen to write every episode.