The Truman Show (1998)
The Unlikely Leading Man: Rubber-faced goofball Jim Carrey, who'd previously spent most of his time in outlandish comedies like Ace Ventura and Dumb & Dumber.
Here, he plays Truman, who's the unwitting lead in a complex reality TV show.
How He Got The Part: It was all down to director Peter Weir, who felt confident that Carrey was perfect for the role.
He was right. Carrey balances the comedy with the inherent tragedy of Truman's situation perfectly, and proved he could handle the lead in something other than a funny flick.
Frankenstein (1931)
The Unlikely Leading Man: Boris Karloff, who slathered on the prosthetics to play Frankenstein's Monster in James Whale's adaptation of Mary Shelley's creepy tale.
How He Got The Part: Well, he just sort of looks like Frankenstein's Monster, doesn't he?
It helped that Karloff was a shave off six feet and didn't mind wearing the platform shoes that added another four inches to his already-towering height.
Casino Royale (2006)
The Unlikely Leading Man: Though it's hard to imagine why the fans were so outraged when "short, blond, craggy" Daniel Craig was cast as James Bond in this series refresh, Craig's casting caused a ripple of outrage in the 007 community.
There was even a website, danielcraigisnotbond.com
How He Got The Part: Though apparently over 200 actors were considered before Craig was cast, the producers felt that Craig was exactly what they needed in order to introduce a grittier, tougher, more rugged Bond.
And boy did Craig deliver, later taking the series to an all-time high with Skyfall 's £1bn payload.
Billy Madison (1995)
The Unlikely Leading Man: "Not an attractive screen presence, he might have a career as a villain or a fall guy or the butt of a joke, but as the protagonist his problem is he creates the fingernails on the blackboard."
That was Roger Ebert's appraisal of Adam Sandler in this goofy comedy, in which Sandler plays Billy, a grown man who goes back to school.
How He Got The Part: Sandler wrote the part for himself, and he's carved a niche out in the comedy genre for goofball funny flicks that are certainly an acquired taste…
Napoleon Dynamite (2004)
The Unlikely Leading Man: Jon Heder, who was initially paid just $1000 to play the titular nerd.
The oddball comedy follows Napoleon's attempt to become high school class president.
How He Got The Part: The part pretty much calls for somebody, um, unconventional, and Heder's curly mop, big teeth and glasses helped him fit the role perfectly.
Fast Times At Ridgemont High (1982)
The Unlikely Leading Man: Sean Penn. Though Brian Backer takes the more conventional role of leading male, Penn's a scene-stealer as perpetually-high surfer dude Jeff Spicoli.
This despite the fact that he looks pretty much nothing like a surfer dude, fake blond locks and sun-cream aside.
How He Got The Part: The beauty of this casting is that Spicoli clearly fancies himself as a surfer even though he clearly isn't one - and Penn's hysterical impression of a wave-rider is spot on.
Scott Pilgrim Vs The World (2010)
The Unlikely Leading Man: Juno and Arrested Development star Michael Cera, who plays the titular Pilgrim - a wannabe musician who's also a bit of an idiot.
He lives in his own videogame reality, and has to fight Ramona Flowers' seven evil exes if he wants to date her.
How He Got The Part: Wright cast Cera after getting hooked on Arrested Development, believing that Cera could pull off playing a character who "audiences will still follow even when the character is being a bit of an ass".
Cuban Fury (2013)
The Unlikely Leading Man: Nick Frost, who's admitted being terrified of dancing.
Which is odd, because Cuban Fury has him starring as a one-time dancing champ who decides to slip back into his dancing shoes in order to woo new boss Rashida Jones.
How He Got The Part: Well, he came up with the premise of the film himself, which probably helped...
The Hangover (2009)
The Unlikely Leading Man: Funny guy Zach Galafianakis, who plays man-child Alan in this bachelor-party comedy.
How He Got The Part: Galafianakis was already friends with other cast members Ed Helms and Bradley Cooper, which meant the actors were able to establish an easy chemistry as their characters.
Also, he's just really funny in this first Hangover , meaning Todd Phillips would have to have been crazy not to cast him.
The Bourne Identity (2002)
The Unlikely Leading Man: Likeable everyguy Matt Damon, whose big screen roles pre-Bourne mostly consisted of him playing gawky young bucks.
Not so with Bourne , in which he transformed himself into an action hero - perhaps one of the most unlikely transformations ever.
How He Got The Part: Director Doug Liman was interested in an action hero who felt like a real, 3D character - which is why he wanted an Oscar-winner in the role.
For his part, Damon was as gung-ho as you'd expect, training for three months and insisting he do most of his own stunts.
