The short answer is... yes.
Privacy laws vary from country to country, but in most developed countries you need to gain permission from a person to use their image in a media project of any kind if they are identifiable in the piece. If the person is considered to be a minor (usually those under 18 years of age) in the jurisdiction where you’re shooting, you will normally need to have a parent or legal guardian sign consent on their behalf instead.
Legitimate news gathering organizations often have special exemptions from these rules as they are generally considered to be operating "in the public interest," but filmmakers (including documentary-makers) do not enjoy this exemption and must therefore get everyone who is recognizable in the film to sign a release form. Without the release, anyone who appears in your film may be able to sue you for using their image without permission, may prevent you from distributing your film, or may even demand a cut of the profits.
If you are shooting with actors or in a controlled environment, make sure you get releases signed at the outset. Leaving it until later on or after your shoot is a very bad idea - what if you lose contact with the actor or have a falling out? It's better to have the releases signed so at least you can use the footage.
If you are shooting on location, always carry some blank releases with you. It's best to try and avoid getting members of the public in your shots, but if it's unavoidable, it's worth trying to get them to sign a release (if you ask nicely, many people won't mind). If you can’t get a release, you'd better forget using that shot.
For documentaries (particularly involving crowds), releases can be a nightmare, but they are normally required none-the-less. Some ticketed live events may include a condition to agree to be filmed in a crowd as part of the terms of issue, but don't rely on this. You'll need to discuss access with the event organizers anyway, so you should also use this opportunity to find out more about the legal implications of filming people in the crowd (something that should probably also be discussed with an entertainment lawyer), like yours truly.
Finally, the other reason why you should get a release for everyone who is identifiable in your film is that most distributors will require you to prove "chain of title" (i.e. that you own all of the intellectual property in the film). This includes rights, which are created during the course of the production in areas such as performance or the images of people you use. In order to prove chain of title, you need to have a paper trail, and that means release forms. Without them, you may severely limit your distribution options as most commercial distributors will not take the risk of picking up films where chain of title isn't complete.
In summary, unless you are an accredited news gathering organization, you must get a signed release form for anyone who is identifiable in your film.