That's the question I answered, along with several other contributors, in the New York Times "Room for Debate" forum earlier this week.
When we go to the movies to see a narrative that is “based on a true story,” certain expectations are set up. Audience members instinctively wonder how much of the film is true. Typically, only a small fraction of those viewers will be curious enough to seek out the real details afterward. The rest will likely take the story at face value, unaware of what sprung from the imagination of the filmmakers.