Being Something They're Not

[Mr. DS]

I started to think the other day about how many films exist out there where the lead character(s) are pretending/known to be something they aren't thus making up the plot line.  Movies like Bean where he is supposed to be a major art expert and Corky Romano where he is supposed to be an FBI agent.  In both cases they're just idiots. I wanted to see how many we could come up with.  

[SPazzo]

In The Man Who Knew To Little Bill Murray thinks he's in a stage-play when he's really involved in a spy conspiracy theory.  (I think, it's been a while).

That's all that I can think of...

[Psycho Circus]

In Point Break, Keanu Reeves "pretends" to be a surfer when he's really an FBI agent. You wouldn't think this is hard, but his acting skills are pushed to the limit even in this role.

Alec Baldwin in Miami Blues is a serial conman, making out he's a banker working the stockmarket and then goes on to impersonate a police officer.

Also, Dustin Hoffman in Tootsie... 'nuff said. 

[Doc Daneeka]

Michael Berryman as Pluto pretending to be an army officer over the radio in the original The Hills Have Eyes... to (un)surprisingly effective results >.>

[Mofo Rising]

Being There - Chance the gardener is taken to be a political genius, when he's really just incredibly simple.

[zombie no.one]

there's an 80s film about a cop that has to go undercover in a highschool pretending to be a student, despite the fact hes virtually gone bald and has to wear a baseball cap to cover it. forgot the name...

[hellbilly]

Tim Allen's character and TV show in Galaxy Quest (1999) mistaken for being real by aliens.

[Mr. DS]

Eddie Murphy in The Distinguished Gentleman

[JaseSF]

Robin Williams in Mrs. Doubtfire comes to mind.

Lou Costello in more than one role I should imagine.

[zombie no.one]

clark kent pretending to not be superman

[SPazzo]

clark kent pretending to not be superman

Well that just brings up all sorts of superhereos; Batman, Spider-man, etc...

[sideorderofninjas]

Catch Me If You Can, where Leonardo DiCaprio, con man, was being chased by Tom Hanks. 
Just One of the Guys, teen girl fakes being a guy in another school. 
New Guy, nerdy kid becomes tough in a new high school...

[Derf]

there's an 80s film about a cop that has to go undercover in a highschool pretending to be a student, despite the fact hes virtually gone bald and has to wear a baseball cap to cover it. forgot the name...

That would most likely be Hiding Out, starring Jon Cryer. Although I don't remember him being almost bald, so it may be another movie you're thinking of. But this one fits, too.

[Mofo Rising]

Probably Plain Clothes, with the balding Arliss Howard. He was cop in that one. In Hiding Out, Jon Cryer is an accountant on the run or something.

[Trevor]

Another example of this is Uwe Boll trying to be a director. Epic fail.