"I'm worried that someone will steal my idea."
Sometimes that thought will:
- prevent you from moving forward on your project
- prevent you from talking to people (some of whom could help you) about your project
- keep you stuck
- keep you in "getting ready to get ready" mode
- make you paranoid and suspicious (Ick!)
And since we want you to be in a state that is clear, flowing and free from psychological impediments, let's see if we can't unpack that anxiety a bit.
First of all: I don't believe that it is possible to steal an idea. But let's try it, just to see. I would like you, right now, to COPY someone else's work. That's right. Pick a piece you like (a book, a painting, a song, a jewelry design -- whatever medium you like) and plagiarize. Copy that piece as closely as you can. Write in the style of that author, sculpt in the style of that sculptor, play in the style of that musician. As much as possible, try to mimic them exactly.
How'd you do?
My guess is that it still came out more like you than like them. (But let me know, OK?)
I will tell you that Elvis Costello actually recommends this practice to beginning songwriters. His theory is that by attempting to mimic the artists you admire, you will both develop your own voice and also learn some of what makes your heroes great.
So even if someone does steal your idea, they can't do with it what you can do with it. Only you can execute your ideas your way.
Secondly, people who steal tend to have pretty lousy karma, and chances are that "your" idea will eventually just turn to dust in their hands. While you, as a creative genius, are a never-ending fountain of ideas! You can just create and create and create. Someone steals your idea? Bless them and let them go. You've got an even better idea coming down the pike right now.
Finally, the fact that more than one person is promoting "your" idea is actually great for business. After all, the Impressionists wouldn't have made nearly as much of an impression if there had been only one of them.
So take whatever precautions seem prudent (register dramatic works with the WGA, ask for non-disclosure agreements when appropriate, check with experts: an entertainment lawyers, literary agents, whomever... ) but please don't keep your work to yourself.
The world needs your art, and if they need so badly that they have to steal it, well, then, perhaps you'd better just give it to them.