Dave

Dave is a 1993 film about Dave Kovic (Kevin Kline), who is hired for a night to replace President Bill Mitchell (also Kline) to cover up an affair. After the president unexpectedly suffers a stroke during sex with his mistress, Kovic becomes the "real" president and gets to work. See the The Other Wiki for the complete plot.

Tropes used in Dave include:

Adam Westing: Oliver Stone and Jay Leno are happy to have a laugh at their own expense.
Becoming the Mask
Bluff the Impostor: How the President's wife confirms Dave isn't who he says he is.
Body Double: Dave for President Mitchell.
Cassandra Truth: Oliver Stone gets wise to the ruse by comparing minute differences in the two men's facial features. Of course, being such a notorious Conspiracy Theorist, no one believes him.
Celebrity Impersonator: Dave was making a living impersonating the President even before he was picked up to do this "gig".
Collapsed Mid-Speech: Dave invokes this. He gives a big speech to congress and the senate, then stutters, stumbles and collapses just after the important confession part, supposedly with a stroke so they can switch the real president back in.
Crouching Moron, Hidden Badass: At first Dave is very silly and doesn't appear to be very serious about being the fake president, just like his puppeteers want him to be. Then he starts actually working to help the nation, and becomes the better president.
Decoy Leader
Dysfunctional Marriage: This is the reason why the Manipulative Bastard dares to pull this off - the real president and his wife are so estranged that they barely talk anymore to each other. Unfortunately, no one counted on Dave being a much better match for Mrs. Mitchell than her husband...
Earpiece Conversation: Dave is coached through two conversations by his handlers and mistakes their stage instructions for his next line both times.
Heel Face Turn: Alan Reed.
Hoist by His Own Petard: The Chief of Staff aims to manipulate Dave into following his own agenda, which will end with him being President. Unfortunately, he doesn't count on Dave both rebelling against his agenda and realizing that as far as everyone is concerned, Dave is the President.
Identical Stranger: The whole plot is this. Justified, since Dave imitated the president a bunch, and that's how the government found him.
Jerkass: President Mitchell, the real one.
Large Ham: Frank Langella as the scheming Chief of Staff, but being a Capra-esque film, it comes with the territory, and Langella gleefully chews the scenery.
Leno Device
Meaningful Name: Mitchell's full name is William Harrison Mitchell, and the screenwriters have confirmed that he was named after William Henry Harrison. Who, by the way, was the first American President to die in office.
Out with a Bang: Kind of. President Mitchell has a stroke and falls into a coma while having sex. He doesn't actually die until later.
Portmanteau
Power Fist: Arguably the telepresence robot arms with a 40 foot reach.

Dave: I once caught a fish this big!

Sexy Secretary: Laura Linney!
Something They Would Never Do: The First Lady first gets suspicious of Dave when she catches him checking her legs out; the real President stopped years ago.
Twenty-Fifth Amendment
Whole-Plot Reference: The plot is essentially an adaptation of The Prisoner of Zenda (a close physical double of a national leader stands in when the real leader is incapacitated, and ends up being both a better leader and a better person all round, falling for the leader's wife in the process), only modernized and stripped of most of the "swashbuckling adventure story" aspects.
Your Costume Needs Work: Towards the First Lady, when she masquerades as a celebrity impersonator of herself.