Being Something They're Not

[Jack]

Plenty of movies where aliens take human form to blend in with society:  Candy Stripers, The Puppet Masters, They Live, The Hidden, Decoys, etc.

And the old cyborg pretending to be human trick:  Crash and Burn, Blade Runner, the Terminator movies, Alien, etc.

[zombie no.one]

clark kent pretending to not be superman

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

yeah, fair point..

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.

it's not that one, same era though...possibly something like 'Back to School', or 'Back in class'?

[D-Man]

There's that one 80's movie, Soul Man, with C. Thomas Howell pretending to be a black man to get a Harvard scholarship.  Though, he really just looked like a white man with darker skin, and still managed to fool everyone, including Rae Dawn Chong, and James Earl Jones.   

[Neville]

- The hidden, with Kyle MachLachlan impersonating an FBI officer.

- A simple plan, where somebody else also pretends to be with the FBI.

And many WWII movies have the main characters impersonating Nazi soldiers or Gestapo at some point. Where eagles dare and Von Ryan's express come to mind.

[Flick James]

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.

Yep, that's the one. Ah, the 80's. Arliss Howard is the guy who was Cowboy in Full Metal Jacket. Looked like an obvious 35-year-old man trying to act like a high school student. Part of a whole slew of high school films about alternate identities, switching identities, anything about pretending to be something one is not. Other examples:

Just One of the Guys (1985)
Can't Buy Me Love (1987)
Plus others brought up on this thread.

Let's not forget also the slew of "switcheroo" films of the same era:

Like Father Like Son (1987)
Vice Versa (1988)
18 Again! (1988)

Ah the 80's.

[metalmonster]

In The Movie EVERYTHING YOU EVER WANTED TO KNOW ABOUT SEX BUT WERE AFRAID TO ASK


The Segment Where The Guy And His Wife Visit Their Friends And He Goes Upstairs Into The Woman's Room And Starts Dressing In Her Clothes , And When She Starts Coming Into The Room He Jumps Out The Window And Pretends To Be An Old Lady Waking Down The Street , And He Uses The Scarf To Cover His Moustache

[Doggett]

Ed in The Man Who Wasn't There*



*could just be my interpretation of the film.