If video-taping in public spaces, you generally have the legal right to video tape people. However, if they appear uncomfortable or signal they do not want to be video taped, please honor their request. If you want to focus on particular individuals for close-ups, please go up to them and let them know your intentions. Again, please honor their requests.
If video-taping a public lecture: let the organizers know beforehand that you'd like to film. Make an announcement to the gathering before it begins that you'll be filming. Suggest that if anyone is uncomfortable with being filmed, they should stay in a specific section of the venue where you won't be video-taping.
If you interview people, please let them know what you intend to do with the materials and have them sign a consent form.
If you intend to "publish" your documentary video-work either through screenings or online, please notify those featured in your work and have proper consents (this is not necessary for people whose images are "backdrop").
When filming or taking photos in public spaces (parks, sidewalks, malls, etc), you generally have the legal right to take photographs of people including children without express consent (unless said people have "secluded" themselves in ways that suggest an expectation of privacy – i.e. in public restrooms, medical facilities, at an ATM machine, etc).
If you are on public property, you can take pictures of private property (i.e. a building visible from a sidewalk). Although there may be areas where taking images is prohibited by law for security reasons, i.e. certain government or military installations).
If you are on private property and asked not to take pictures or video tape, you are obligated to honor that request and may be asked to leave the property.
Legal rights to photograph or film are stronger than legal rights to publish those images. Although you may have a legal right to take a picture of someone in public, you cannot publish that picture for commercial gain, for example, without their permission. (i.e. television commercials, ads, and promotional videos are treated differently and have less protected rights than filmmakers, documentary filmmakers, and news crews). Also, if the images are used in a potentially defamatory way, you could be subject to a lawsuit.
These are just guidelines and not legal advice per se. Laws vary by state and between countries. For more information, see the Legal Handbook for Photographers by Bert Krages.