I've now seen several different trailers for Jim Carrey's upcoming release Eternal Sunshine of the spotless mind.  The initial teaser struck me as bizarre and a little bit darker than I usually like.  But then, out came the full trailer, and all of a sudden I had a better feeling about it, and I am now looking forward to seeing it.  After pondering what could have caused my change of heart, I realized that it was the use of ELO's (Electric Light Orchestra) Mr. Blue Sky in the background.  If you know the song, then you know it's hard for the mood to be dark when that song is happening.  Anywho, I just thought it was interesting that a song choice for a trailer can really change a lot.