If Life Was A Musical, Who Would Write The Lyrics?

Getting into 18A movies nowadays is rally easy. They don't eve ask for ID... I mean, this poses a serious security issue. The girl I was with wore her 18A attire (major cleavage), which I suppose helped. I think I look about 19, and I didn't even have to say 'she's with me'. The dude at the door ripped our tickets and let us in. Poor bastard's really bad at his job.
ANYWAYS, we went to go see Tim Burton's new gory flick - Sweeney Todd (which was, by the way, 18A for a good reason). It was uber good. Johnny Depp never fails to deliver and Helena Bonham Carter was superb. However, there were a lot of people from Harry Potter in that movie, it was almost like a side story JK Rowling decided not to write. How weird. And when I say "people", I do mean actors.
I loved the overall story, atmosphere, acting - everything except (yeas, there's always an 'except'), it was a musical. Which in its own way is original and ironic considering the nature of the movie's plot, but it posed another serious question: are the characters aware that they are singing, or are they simply talking and in some parallel way they are singing? Weird stuff, dude. This is something that just came to mind as I sat there staring into the black abyss that I call my life - the movie screen just happened to be in the way.
If life was a musical, who would write the lyrics?