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Alas, despite earlier reports that Mel Gibson would have a cameo appearance in 'The Hangover 2,' he's now been dropped from the cast, apparently owing to objections by Zach Galifianakis and other cast members. Too bad; the jokes would have written themselves.

But then, that's the point, isn't it? We'd have gotten see Mel Gibson laughing at his own misconduct, he'd have proved he's a good sport, he'd work his way back into our good graces. Meanwhile, the publicity/shock value would have gotten a few more people to buy tickets. The whole process worked so well with Mike Tyson's cameo in the first 'Hangover,' and there's no reason to think it wouldn't have worked again with Gibson.

There are a lot of reasons filmmakers seek stars for cameos, and a lot reasons stars take on cameos, and those reasons usually have little to do with serving the story. Some of the reasons are outlined below -- along with some of our favorite cameos.

1. The "Career Rehab" Cameo. 'Hangover' bit player Tyson isn't the first celebrity to use a movie cameo as a way to jump-start a stalled career or rehabilitate a tarnished image. After his 1980s sex scandal, Rob Lowe spent much of the '90s making cameos in Mike Myers movies to prove he was incapable of embarrassment, and funny to boot. He played an oily corporate heavy in 'Wayne's World' and showed off his dead-on Robert Wagner impression as the younger version of Wagner's character in 'Austin Powers: The Spy Who Shagged Me.'

Fellow Brat Packer Demi Moore made a comeback after a long screen absence with a highly publicized cameo as a villain in 2003's 'Charlie's Angels: Full Throttle.' That same year, Corey Haim and others made humorous, self-parodying cameos as fellow washed-up young actors in David Spade's 'Dickie Roberts: Former Child Star.' Britney Spears is another skilled user of the career-rehab cameo, albeit usually on TV guest spots ('Will & Grace,' 'Glee'). Hey, it's easier than spending a week working on self-lacerating satirical sketches as a guest host on 'Saturday Night Live.'

Stephen Baldwin had a good one of these a couple years ago in 'Fred Claus.' Like the title character (Santa's ne'er-do-well brother, played by Vince Vaughn), the youngest Baldwin shows up as part of a support group for guys overshadowed by their more famous brothers. Stephen gets laughs when he throws a fit while applying his misdirected jealousy of Alec Baldwin to Fred. It works for the story, but it also serves to remind viewers that the 'Bio-Dome' star is, despite recent career doldrums, still a pretty funny comic actor.

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2. The "Audience Surprise" Cameo.
Some stars are so much bigger than their tiny roles that their very appearance tends to throw the picture off balance; but then, that's part of the intended effect, which is the reason their appearances are usually spoilers best left undiscussed. The king of all such cameos is Sean Connery's appearance as Richard the Lionhearted at the end of 1991's 'Robin Hood: Prince of Thieves.' The fact that it's Connery gives the character an instant air of authority and reverence that the screenplay cannot, though his presence is so grand that he pretty much steals the movie from Kevin Costner (so it's good that he doesn't show up 'til the end). A more recent example is Bill Murray's surprise cameo as himself in 'Zombieland,' which pretty much made the movie for many fans.

One of my favorite surprise cameos also comes from a Mike Myers movie. In 'Wayne's World 2,' a gas station attendant is telling a poignant story, but the scene plays flatly. Myers breaks character and calls for a better actor. The scene is reshot, but with Charlton Heston as the pump jockey. The scene is moving, everyone mists up a little, and the movie proceeds.

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3. The "Favor to the Director" Cameo. It's not like 'The Expendables' really needed that sequence with Arnold Schwarzenegger; nor did the Governor of California really need the cameo as a way to dip his toes back into show business. But it made sense, both as a tribute to aging '80s action heroes (along with the rest of the movie) and as a favor to Schwarzenegger's Planet Hollywood business partner, 'Expendables' director-star Sylvester Stallone.

Of course, the cameo-as-favor can work both ways. Clint Howard has an acting career primarily because brother Ron keeps giving him cameos in the movies he directs; same with Ted Raimi, who frequently cameos in brother Sam's films. Sam Raimi also keeps giving bit parts to Bruce Campbell (in the 'Spider-Man' films, for instance, he's a wrestling-ring emcee, a theater usher and a maître d'), as if to acknowledge that the director and actor put each other on the map via the 'Evil Dead' movies. Rob Schneider's movie career has been hit-or-miss, but he'll always have a place in buddy Adam Sandler's films.

Members of the so-called comedy Frat Pack often appear in brief roles in each other's movies. The best example of this is probably 'Anchorman: The Legend of Ron Burgundy,' with its notorious alleyway rumble that sees stars Will Ferrell, Paul Rudd and Steve Carell brawling with rival newscasters played by the likes of Ben Stiller, Vince Vaughn and Luke Wilson.

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4. The "I'm the Director" Cameo. Alfred Hitchcock was the pioneer of this technique, as one of his signatures was a blink-and-you-'ll-miss-it walk-on appearance in nearly all of his films. Oliver Stone frequently gives himself a bit part in his own films as well. Francis Ford Coppola can be spotted playing a war documentary filmmaker in his own 'Apocalypse Now.' Quentin Tarantino cameos as one of the thieves in 'Reservoir Dogs' and as the friend who lets the blood-spattered John Travolta and Samuel L. Jackson clean themselves up at his house in 'Pulp Fiction.'

Watch Steven Spielberg in 'The Blues Brothers'

John Landis often enlists other directors for cameos in his films, like the county tax clerk played by Steven Spielberg at the end of 'The Blues Brothers.' In Landis's 1985 thriller 'Into the Night,' there are 17 director cameos, including David Cronenberg, Amy Heckerling, Roger Vadim, Jim Henson and Landis himself. M. Night Shyamalan gave himself cameos in 'The Sixth Sense,' 'Signs' and 'The Village' before taking on a major supporting role in his 'Lady in the Water.' Peter Jackson played unrecognizable warriors in walk-ons in the three 'Lord of the Rings' movies.

Martin Scorsese often cameos in his own movies, including one of the best-ever cameos by a director in his own film: the creepy passenger in 'Taxi Driver' who talks about killing his wife to the similarly disturbed stranger behind the wheel, Robert De Niro's Travis Bickle.

5. The "Nod to the Fans" Cameo. In these cameos, the very casting itself is an in-joke. It takes viewers completely out of the movie but lets them pat themselves on the back for recognizing the director's winking acknowledgement of their fandom.

Nearly every film adaptation of a TV show does this by giving the original TV stars a token role in the film (like Paul Michael Glaser and David Soul showing up at the end of 'Starsky and Hutch' to pass the torch by selling the program's famous car to Ben Stiller and Owen Wilson). Adaptations of books do this, too, which is why Stephenie Meyer can be seen dining alongside her creations in a restaurant scene in 'Twilight.' Marvel Comics fans all know publisher Stan Lee, who has a walk-on in nearly every Marvel superhero movie, including the 'Spider-Man,' 'Fantastic Four,' 'X-Men' and 'Incredible Hulk' franchises. Then again, only the most hardcore S.E. Hinton fan probably knows what the 'Outsiders' author looks like, or would recognize her as a nurse in the 1983 film version of her novel.

Watch Every Stan Lee - Marvel Movie Cameo

When a movie is based on a true story, the real-life person often lands a cameo in the movie. While Julia Roberts is playing the title character in 'Erin Brockovich,' she eats a meal served by the real Brockovich, playing a waitress named Julia. Legendary test pilot Chuck Yeager shows up as a bartender serving drinks to the fictional flyboys in 'The Right Stuff.' In 'Almost Famous,' which features Rolling Stone publisher Jann Wenner as a character, the real Wenner appears as a passenger in a taxi.

Best nod-to-the-fans cameo was probably Leonard Nimoy as the original Spock in last year's 'Star Trek,' since the screenplay actually devised a good plot reason for the elder Spock to appear. His presence worked on the movie's own terms, as well as providing a way to acknowledge 40-plus years of 'Star Trek' fandom and to pass the torch to new Spock Zachary Quinto.

6. The "Everyone Else Is Doing It" Cameo.
Think of the dozens of Hollywood stars playing themselves in 'The Player,' or the dozens of Washington politicians playing themselves in 'Dave.' Get enough cameos like these in a single movie, and you can create a whole world with a sense of verisimilitude.

Of course, sometimes, getting a boatload of cameos is done simply to prove a filmmaker is powerful enough to get a boatload of cameos. The term "cameo" first became popular outside showbiz circles with the release of 1956's 'Around the World in 80 Days,' a movie with about four dozen cameos in it (including such stars as Frank Sinatra, Marlene Dietrich, Noel Coward, Peter Lorre, Buster Keaton, Jose Greco, Red Skelton and newscaster Edward R. Murrow), whose poster advertised many of the cameos via oval, brooch-shaped portraits of the stars. They weren't there to serve the story, but to serve producer Mike Todd's effort to make one of the biggest, grandest spectacles in Hollywood history.

Another powerful producer, Stanley Kramer, performed a similar feat a few years later, getting pretty much every comic performer in Hollywood to appear in his overstuffed comedy spectacle 'It's a Mad Mad Mad Mad World.' Also from the early 1960s, John Wayne's 'The Alamo' and George Stevens' Jesus biopic, 'The Greatest Story Ever Told,' were full of countless cameos. And the trend continued in the 1970s with producer Irwin Allen's disaster movies ('The Poseidon Adventure,' 'The Towering Inferno'), which gave numerous starlets and old Tinseltown veterans alike the chance to play an elaborate death scene.

Watch Robert Wagner Go Up In Flames in 'The Towering Inferno'

These days, about the only occasion where you're likely to find so many stars in one place at one time, checking their egos at the door for a chance to appear in a project fulfilling someone else's grand vision, is at one of George Clooney's disaster relief telethons. There aren't too many cast-of-a-thousand-cameos movies anymore, which is too bad. A cameo may be a tiny role, but sometimes in the movies, more is more.

•Follow Gary Susman on Twitter @garysusman.