I think it is always a bit of a gray area when it comes to that. How much of the idea is changed so it is not directly referable to your idea. I am no expert on this topic, but there are ways of protecting your IP. However I think that in for example, A mario clone that does exactly the same as mario, but has different character and such is perfectly fine in terms of law (to a certain degree, you can't just name is dudebro and change only the main character).

The best solution is probably find someone you know you can trust, that doesn't walk away with your idea. I don't think that you can fully stop someone from stealing your idea should it happen.

first off ignore stormynatures post. Copyright has no relevance in protecting ideas. Copyright law concerns the right to reproduce original creative works, If you copyright a game design document all the gives you is protection if someone were to reprint in for a example a book or website without your consent. The ideas themselves and any can legally create their own version as long as they don't reuses any of your creatives(text, images,audio)

What you are looking for is normally covered under a combination of an NDA and employment contract. If you are planning on doing any real work with these people and are worried that you have the next big idea then you'll want to enter into a legally binding contract with them. Most employment contract will contain a work for hire agreement that states that anything the employee does is owned by company and not the employee, which ensures anything produced while working on the project is owned by you.

The contract should clearly state what you are expecting from the other person and what they are getting in return.

Keeping files on the different projects being worked on is also import, as if issue goes to litigation you'll have to prove that a concept someone stole from you was something they were involved inn while they were under contract.

An additional note-- once you have something fixed in a tangible form, whether it be a script or prototype, you will be running the risk of being joint authors/owners of the work. If that happens there's little you can do to prevent them from exploiting the work, since joint ownership in IP is tantamount to being tenants-in-common. In other words, you'll each own an undivided interest in the work. At that point either party can, with the exception of assignment or exclusive licensing of the entire work, do whatever they want with it. This includes selling off their interest in the work.

To avoid this you'll want to ensure that any agreement you enter into has an IP control provision. This could be a work for hire, assignment, or rights management provision that limits how the other party can exploit the work both prior to or after the release.

You'll need to be careful with non-compete provisions, as excessively broad non-competes are invalid in most jurisdictions. In fact non-competes in general are invalid in many states, so it's important that your contract is governed under a jurisdiction that recognizes this constraint.

Finally, it's important to specify in detail what constitutes confidential information. The most problematic aspect of upholding an NDA is the fact that the other party may be unaware of what is confidential versus what isn't, so you need to make it clear in the agreement itself that correspondences relating to ideas concerning the game's production, both oral and written, are confidential and the disclosure of those ideas will lead to economic harm.

How much of this is a hobby, and how much is it business - e.g., will you be paying someone to do this?

Consider it from their point of view too - who is going to help you with your "idea", if that means signing something that means you can sue them if you later decide any work they do in future has any similarity to your ideas?

I think it's also worth asking yourself if it's worth worrying about, if you're talking about mere "ideas", and not the tangible work (which is worth worrying about).