It depends on what you mean by "fake." If you are using names of celebrities and actual people who did not work on the film, you may have some issues.
If you are creating fake names and positions like is done in many comedy movies then you are likely okay.
This may also violate the terms of the agreements you have with your talent/crew.
You should present your fake credits to an attorney to review for any possible liability that may arise.
Your question is not clear. What do you mean that the credits are "fake"? Are you associating actual people to your project that were not involved? This may get you in legal trouble. Consult with an entertainment attorney.
I agree with my coilleagues that there's ok fake, for the sake of comedy and/or contractual or requested anonymity ("Alan Smithee"), and not ok fake, where you're violating people's rights and/or passing off your film's cast and/or crew as other people.
You're already pushing the envelope with no rating and strong language, so you're in this business for the long haul and are planning for success, see a lawyer ASAP to practice some preventative law and "vet" your film and make sure it doesn't expose you to an increased risk of liability.
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That is OK provided they are original as well as fake. If you are falsely claiming association with some real person, you are illegal. How and how much you sell is mostly a damages issue. If this is a business for you, you need to run the specifics by an IP attorney handling copyrights. Otherwise, some studio people may get offended and even Picatinney Arsenal could not help you.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
I think you've gotten the legal answer from the previous attorneys, but what I'd like to know is why you would want to create fake credits in the first place? The only thing I can think of is for comedic purposes, as mentioned in some of the answers. Is that your purpose?