Commissions are tricky. And unless you have been thorough you might end up working "on-spec". This means that you create a commissioned work, and then, once it's complete, the client views it and decides whether they will purchase it or not. Ouch! This can be a truly unholy, rude awakening. With equal parts foresight and clear communication in written or verbal form, you can effectively ward off evil and reassure yourself that your project is totally worth your time.
"Having a written contract signed by both parties is meant to: eliminate any surprises for both the client and artist (everything is agreed upon in advance) and protect your interests and the interests of the client"