What are the rules for filming in a public place? Is it illegal to use images of specific people if they are only "on screen" for a few seconds?
If you are really in a public place, like out in the street or something, then you can film people no matter how long they are on the screen. However, if you don't have release from them, you cannot use any images of them in a "commercial" manner to advertise or sell the film. So they could be in the film, but without a release, you couldn't use that clip to advertise, or as a trailer, or use stills from that clip on a poster or ad.