Generally, the imagery and the text go hand in hand. It's much easier when the text comes first, but sometimes I need visual stimulation in order to find the words. I get an idea of what I want when I begin to shoot, and the text is usually the last thing to be resolved. I tend to leave the text open, and I refine the words up to the last minute. As for the image, I can resolve that and get that done fairly quickly.
