So Al Gore has been making multimedia speeches about global warning, and the producer Laurie David decided to make this into a movie, recruiting 'Deadwood' producer Davis Guggenheim to direct. Guggenheim persuaded Gore to open up about his personal life to provide the human connection for the film. A quote from an interesting interview about this:
Q. The marketing for the movie -- the trailer and the poster -- are completely over the top. "The scariest movie you'll ever see!" But the movie itself is quiet and methodical, and quite hopeful at the end. Did you deliberately choose those respective strategies?
A. It's a great trailer, very effective. But the people who make the trailer are completely different from the people who make the movie. I think they've done a terrific job on the movie, and I think a different group did a terrific job on the trailer.
The purpose of a trailer is very different from the purpose of a movie. I talked with Steven Spielberg, who saw the movie and loved it, and saw the trailer and loved it. And I asked him pretty much the same question you're asking me.