I have a TOP SECRET invention/book idea/character, etc. that I'm nervous about sharing. How do I proceed?

Some creators prefer to send the artist a Non-Disclosure Agreement (NDA) to sign. This would bind them to secrecy contractually. If this is important to you, feel free to ask the artist to sign an NDA with you. There are plenty of NDA templates online you may copy, or simply write your own Work For Hire Agreement you both will sign that states the ownership of the finished material.