I think everything needs to be clear from the start. If you are looking for someone to capture your bands sound. You pay a studio and engineer to do that. They will probably make suggestions on what might help, but it's your call in the end.
If you have some rough demos and need someone to help you bring them to life, then you need more of a producer. At the time you start a relationship like this, you need to get the cost in writing.
As far as I'm concerned, there is a lot of grey area in money and music. If you feel like you are contributing more than you are getting credit for, then you need to either get your title in writing or risk not getting your due.
Even as a player, you can come in and start playing to some tracks. If something is written for you, you are just playing and will probably get an hourly rate or day rate. If you come in and someone says, "check out these tracks and see what you feel". You might want to ask at that time Am I being asked to write a part? If you are, you probably deserve more.
The system doesn't have set rules (especially if you are building a rep), So you want to get anything that could fall through on paper. You might not realize it until it's way too late and you don't get some points you deserve!
