as I drive a lot for work, I have an idea pop in my head and usually have some lyrics pop right out. that is where the trouble starts for me. as I have an idea about the tune, but I have a hard time getting the tune out or finding a way to play it. if I try and sound out the notes with a guitar or keyboard (note by note) to try and capture what is in my head, I lose the melody.
anyway I seem to have words pop into my mind, then I try and make some music to fit them. pretty much like what they were doing in the movie music & lyrics. although not much seems to work out as well as it did on the movie