One of the biggest problems that can shut down a movie after production has wrapped is the threat of a lawsuit. This is something that can occur at any point in time, and will likely create huge headaches. If you find yourself trying to avoid these problems you will have much better luck ensuring that your films actually are released, rather than being trapped in the holding pattern. Taking the time to avoid potential legal problems in the beginning as you are working on the script will go a long way towards avoiding these problems.
When you are working on your script, you need to ensure that you are basing all characters off of complete fiction to avoid all problems. If you really want to base a character on someone that is real, you should take the time to obtain their written permission. However, if you really work at adjusting the character you should be able to remove most of the differences. Additionally, when you are naming characters you should check in the location where you have set the feature film to ensure that nobody has that name there. For example, if you are creating a film that is set in Omaha, Nebraska and are planning to name a character Otis Smith you should look in the Omaha area to determine if anyone has that name.
If you find someone who does have that specific name in the area, you have two different options. You could either change the name, or you could change the location where the movie is based. If you are set that the name needs to remain then changing the location is best. If you are set on the location then your best option is to change the name. The ultimate choice is up to you, but this will help you to prevent anyone from ever saying that you have taken their life to use in your feature film.
As an added step for your film, when you are editing you need to take the time to add in a small disclaimer at the beginning. The disclaimer should just plainly state that all characters are purely fictional and that any similarities to anyone living or deceased is merely a coincidence. This will help to protect you legally in the event that someone tries to make unfounded claims against your film.
If you are considering using fictional characters that someone else has created for your film, you absolutely must obtain permission in writing. Without the permission, you will almost certainly find yourself the victim of a lawsuit. Depending on the characters that you are interested in using you may have a bit of trouble obtaining permission. The higher the profile of the fictional characters the more important the permission is. If you are interested in using only limited rights to the characters, make sure all of the details are laid out clearly in a contract that is signed by both yourself, as well as the creator of the characters.
Taking a few steps to protect yourself, as well as your film will ensure that all of your hard work is recognized. Working on a film and discovering that you are not allowed to release it to the public is a crushing blow to many filmmakers. Taking the time to avoid such complications will only take a brief amount of time, and frees up your attention for other matters.