The Birds (1963)
The Cameo: It's hard to pick just one of Hitchcock's many fleeting appearances, but it's this one that has become his most iconic - by virtue of sharing the screen with a pair of small dogs.
If The Character Had More Screen Time: Hitch would play an animal expert called in to solve the birdemic... But who inevitably gets pecked to death.
X-Men: First Class (2011)
The Cameo: Having long established that Wolverine doesn't age, this 1960s-set prequel has fun having Hugh Jackman tell the young Professor X and Magneto where to shove their plans.
If The Character Had More Screen Time: A major continuity headache, given that he doesn't join Professor X's lot for several decades.
Hot Shots! Part Deux (1993)
The Cameo: As Topper Harley (Charlie Sheen) heads upriver, he passes Captain Willard (Martin Sheen) heading the other way - a cute in-joke considering father & son's shared 'Nam heritage in Platoon and Apocalypse Now . Cementing the joke: "I loved you in Wall Street !"
If The Character Had More Screen Time: It wouldn't be beyond the realms of Hot Shots ' implausibility for Sheen, Snr to tag along with Charlie for the rest of the film.
Annie Hall (1977)
The Cameo: The medium is the message, so when Woody Allen can no longer cope with a clueless academic, who better to do the take-down than postmodern guru Marshall McLuhan? The highbrow version of getting pwned.
If The Character Had More Screen Time: Eventually, McLuhan would start deconstructing Allen's own comedy and Woody would be reduced to a gibbering wreck.
Pulp Fiction (1994)
The Cameo: Christopher Walken gets 'The Gold Watch' off to a gripping start simply by reciting a monologue of wartime imprisonment and anal discomfort in his trademark stilted delivery.
If The Character Had More Screen Time: He'd still be hanging around as Butch's (Bruce Willis) guardian angel, coming to the rescue in the gimp's cellar.
The Cameo: "Do you like scary movies?" Unexpectedly killing off your most famous actress isn't exactly a new gimmick, but Drew Barrymore's bite-sized appearance distils things down to a cameo-length scare.
If The Character Had More Screen Time: You may as well drop the slasher element entirely and make a high school comedy.
The Cameo: Survivors of the zombie apocalypse hole up in a Hollywood mansion, only to find that the owner - Bill Murray - has disguised himself as the undead in order to continue playing golf.
If The Character Had More Screen Time: We'd be robbed of one of the freakiest meta-jokes in cinema, as Murray is mistaken for a zombie and shot. His deathbed confession: he regrets doing Garfield .
The Cameos: Ron Burgundy's crew face off in a violent confrontation against rival news anchors - including Vince Vaughn, Luke Wilson, Ben Stiller and, improbably, Tim Robbins.
If The Characters Had More Screen Time: It'd become an epic tale of the San Diego television industry. Think Mad Men , with trident murders.
Tropic Thunder (2008)
The Cameo: Tom Cruise's appearance as overweight, balding, bad-dancing studio exec Les Grossman was so shrouded in secrecy that Internet sites publishing leaked photos were handed 'cease or desist' orders.
If The Character Had More Screen Time: It'd quickly get wearying if Cruise had to be that OTT for a whole film. That said, there's scope for him to crop up in other films about Hollywood.
Glengarry Glen Ross (1992)
The Cameo: Not everyone could upstage Pacino, Lemmon and Spacey, but Alec Baldwin knocks it out of the park playing a character specially created by David Mamet for the film version of his play. Remember - Always. Be. Closing.
If The Character Had More Screen Time: The intensity would be as exhausting for Baldwin as it would be for us. Better by far to leave Baldwin with the honour of cinema's greatest cameo.