The short answer to this issue is, yes, you must obtain permission before using an actors image in film or marketing material. You must get each person who is identifiable in your film to sign a release allowing you to use their image in the film and associated activities (such as marketing materials for the film). This applies equally to narrative films and documentaries. Regardless of how you are planning to use the film or whether you intend to charge people to see it or not, you must be able to prove that you hold all of the underlying rights, including script, music, and the right to use your actors' images.
If you've gone ahead a shot your film without getting your actors to sign releases, you should try and get them signed retrospectively as soon as possible. This is probably not going to be a problem if the actors are your friends, but if your relationship with them deteriorated on set you might find it harder to secure agreement after the event (hence the reason why it's essential to get them signed ahead of time). It's certainly far to risky to use an actor's image without their consent, particularly if you film ends up earning any money.
The upshot is, always always always get your releases signed in advance!