Don't.
The truth of the matter is that the idea itself is not the object of value. As you've heard before, the long term execution is where the value lies. You could spend money and stress on drawing up an agreement, which is not enforceable for free, but that time is better spent working on the project.
The risk of somebody stealing the idea is less than it seems at first because the distribution of people is not uniform. If you don't select a person because they are incompetent, they don't have the ability to do anything with the idea. If you don't select a person because they are dishonest, a piece of paper will not stop them from attempting to steal the idea. If the idea has any differentiation from publicly available ideas, that will likely be found in details uncovered throughout the process that won't even be discussed during your meeting.
In a sense, you would only end up trying to protect yourself from the guy you would likely choose anyway. He is competent, enthusiastic and honest. Worse, you may scare a guy off from an informal interview by turning it adversarial at the beginning.
I am not suggesting that there are not dishonest, nasty people in the world, only that it is not the best use of your time to try and foil their evil plans.
