An imposter is one way to put it. A poser is a person who is not authentic about themselves. A person who pretends to be something they're not can also be delusional. A fraud, fake, poser, pretender, sham, charlatan
There are probably lots of movies like that. One that comes to mind is The Secret of My Success, where the character played by Michael J. Fox actually is an employee... in the mail room. However, he pretends to be an executive.