The digital world has put a camera in a lot of hands that wouldn't have been able to make a movie in the analog and film days of the not so distant past. But, before you go off half-cocked; there are a few things to consider before you start making your masterpiece.
If you're making a truly original movie; another words you wrote it, you have less to worry about. Yes, you have issues to be aware of, such as unauthorized use/exhibition of products and lack of pemission to exhibit the person or likeness of someone. But, at least it's your baby.
If you're making a movie of someone else's script, then you'll be needing a legal document setting forth the terms upon which you are able to do that. This agreement will include representations made by the author that no one's rights have been violated. This becomes even more important after you make the movie and have to make the same promise to the distributor taking your film to market. You also want to be sure that you will be the one copyrighingt the movie itself, even though the author owns the script copyright (unless you've purchased that as part of your deal).
So now you have your "intellectual property" in place. You'll need actors. You'll need crew. You'll need equipment. You'll need locations, props, supplies, anti-acids; the list is practically endless. Getting all of this together involves alot of legal relationships and liability, deal memos, contracts, union rules (maybe). Sure, you can and should get insurance, but do you really want personal liability for all these things?
No, you don't. Form a company and transfer rights to make the movie into it. You can still manage the company and maintain control. But, the company will have the legal responsibility for the people and things involved in your process. That's a big deal.
How are you, well, now it's your company, paying for the production of your movie? If you're paying for it, God bless you. If someone else is paying for it, or helping you pay for it, you need to ask yourself what other role the person or people are playing in the production (besides providing $). Sorry, we're talking participation in the process, not just the actors playing their roles - don't want to confuse anyone more than necessary.
As a general rule of thumb, if the investing person is actively involved with you in making the movie - and - in making decisions in a meaningful way about how the investment is being utilized, then you have what we securities lawyers call an "active investor."
If you're getting the money from someone who's sitting back and waiting for the movie to be made and distributed (so he, she or it can at least recoup investment), then you have a "passive investor."
This opens up a whole bunch of other legal considerations that boil down to - you need to comply with securities laws and regulations in taking this person's $. Please, if you find yourself in this situation, consult with a knowledgeable attorney. In this business, an ounce of prevention can be be worth a ton of cure. Keep on truckin'!