All of the above advice is right on. The more you know what others do that works well, the stronger your starting position. Some experiential points …
We used as resource several online release forms to fashion the right one for each project, and sometimes use different releases for different subjects within one project. Some are highly legalese with hundreds of words (corporate VIP or institutional subject speaking on behalf of organization, etc.), and some are very simple, just a few paragraphs (person on the street, etc.). Grabbing the opinion of the perfect stranger at a public location who has a great sound bite is wonderful serendipity– but whip out your 1,000-word release and you may kiss that moment goodbye. For all versions, must have a lawyer do a looksee and tweak.
For a location like a restaurant, the owner or legal rep of the location must sign a location release. And if you post a sign at the entry point(s) of the place stating that those who enter may be filmed for a documentary, etc. you are more covered. If you zoom in on some customer and create some good specific background action with him/her, then get a release.
As far as strangers who end up in a sequence or scene, if it is not practical or possible to get a release make sure you have some coverage of the action that allows you to use the scene but not see the person’s face for longer than a second; get some over the shoulder, frame their hands, etc. And also don’t use their voice as a principal source of info, keep it short and light.
But there is such a thing as fair use when you are shooting a documentary, after all.
The bottom lime is, as said in post above, anybody can sue anyway. Study recent cases for “Borat”, which had gone as far as to purposefully humiliate strangers. They also had no releases for certain passersby on the street and subway. Although this film was seen by many millions worldwide, the issues of fair use and what constitutes “news” are interesting.
Ultimately, the proof of your intentions during the shoot and for the final material are important.
Good luck!