Is it possible to bluff one's way to the flightdeck like in Catch Me If You Can?
A lot has changed since the Frank Abagnale / "Catch Me If You Can" days, and riding as a non-revenue passenger (particularly in the cockpit jump seat) is no longer not as simple as grabbing a passable captain's uniform, polishing your shoes up, and smooth-talking the flight crew.  

Someone seeking a ride in an airplane jump seat would still have to pass through airport security and prove their identity &amp; authorization to occupy such a seat to the satisfaction of the crew operating the flight, which would likely include checking credentials that are at lease somewhat difficult to forge.

So in short it's possible – if someone is particularly adept at social engineering, forgery, and has at least a working knowledge of airport &amp; airline security procedures they might get away with it – but it's extremely unlikely, and as has been pointed out the consequences if and when you get caught are particularly unpleasant these days.