One of the important aspects of knowing your co-writers' strengths is so that you know who would be best suited to finish any given idea you come up with. I would encourage you to keep an organized list that you can look to the morning of a co-write. Every co-writing relationship is different, but I find it usually helps to have some idea that launches the creative process into motion.
In some ways, it's like you get to play Santa Claus with a big magical sack full of song ideas.
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