How do you go about it? What's your approach? Do you come up with an idea for a story or a theme, or a turn of phrase which you want to set to music, or do you get a tune in your head first and then set words to it?

I've recently found a musical partner to write with, but our styles are quite different. He needs to be sitting in front of his piano or with his guitar and he expects it all to come toggether at once. I don't mind sitting at the piano/ with guitar and noodling around but it only works for me if I have an idea of what the words are doing first. And the words usually come to me when I'm on a bus or a train (i'm that dick feverishly writing into their moleskin and biting their fingernails off on public transport.)

How do you do it? And how do you reconcile different styles with other people?