Roll when the players arrive somewhere and you need something to happen there. Who is there is a manner, not a name: decide who they are once you see what they want. The complication is what turns the scene. Keep what fits and roll the rest; Copy puts the encounter on your clipboard.
For an ALRP author one encounter can be a whole post: end it at the complication and let the players send in what their characters do. In a shared gameworld, the people at a tabletop encounter can be the ALRP players' characters (Playing with Tabletop Gamers).