Firstly, you'd have to assume that there is no primary sources from today that would explain the context of the movie.

I think a better example would be the 12 Caesars by Suetonius. It's packed with definitely real people and Suetonius was close to Hadrian -- so it's really an invaluable resource to historians. However -- it's filled with gossip and dubious claims.

As a historian, you can't discount Suetonius just because he fudges some of the details -- he was too close to the action! So, if you see something that seems extraordinary, what you'll want to do is try to find some corroborating evidence. Suetonius is the one that claimed Nero played the lyre while Rome burned, but there's virtually no evidence of this, so historians discount this as gossip -- his way of stressing how goddamn crazy Nero was.

A historian works sort of like a reporter working on a story where nobody is going to talk. So, say 5,000 years from now you find this movie from a few years after World War II (in 5,000 years, 2009 would seem to be really just a few years after the war), telling this amazing story about the American Lieutenant during the war.

Now, you know for damn sure he didn't kill Hitler in France, but maybe he existed? The only way to verify that would be to find a second source. You'd assume he'd have to have left quite a mark for people to make a movie about him, but you'd come up with nothing that predates 2009.

There's a chance they could draw some inaccurate conclusions -- instead of being just a made up character you might conclude he was an amalgam of several different war heroes, but you'd likely conclude that a man named Aldo Raine never actually served in World War II.

Now, that would be historians. But, if you asked a random person on the street to tell you something about Nero, they'd probably mention that he played music while Rome burned. So maybe, 5,000 years from now, Aldo Raine will have lived for everyone except historians.

For a long time, one of the most famed strategist of the Sengoku era in popular folklore was assumed by historians to be fictional. Yamamoto Kansuke was an extremely brilliant adviser to Takeda Shingen as recorded in the Kōyō Gunkan. The Kōyō Gunkan however, based on comparison with other sources, we know is not a very good narrative history, filled with many errors. As all records of this brilliant strategist was based on this one source and there was no other record mentioning such a person, it was assumed he was made up, or the author purposely exaggerated the role of an ancestor/relative as to make a legend.

A few decades ago, during the making one of NHK's annual historical drama (for some reason the annual drama seem to lead to many historical discoveries), a letter of Takeda Shingen surfaced that confirmed his existence. Since then, a couple other records have appeared that confirms his existence and demonstrates that he was close enough to Takeda Shingen to be employed as an aide/diplomat. The brilliant strategist part is still out in the open though.

I think that's kind of how historians would deal with a case like this. They would compare this source to other known sources and evaluate accordingly. As new sources emerge, old theories would be re-evaluated.