My coping mechanism for interacting with people who approach me is: 1. Act friendly and open when they start talking to me; 2. centre the questions on them, hoping that they reveal information that might clue me in on who they are. It's only in recent years that I've learned step 3: if I haven't recognised them in the first 10-20 seconds, mention that I'm face blind (usually as an apology) and ask them who they are. Most times that is enough to stop the encounter turning sour.
My coping mechanism for approaching a person I think I might know (usually in spaces where I wouldn't expect to encounter them) is: don't. At least not immediately. Rather, watch and observe (if possible) to see if the voice/gestures/body positions/etc firm up enough to bring the certainty levels up and the risk levels down.
I think I'd make a good spy, if only I didn't suffer from this face blindness nonsense.