Here are a few notes I took when I was studying up on acting prior to directing In-World War. I just noticed I never posted my notes so here they are. These apply to any type of acting really: feature film, independent, DIY, theater, etc.
1. Listen (react don't act). The reaction shot is key.
2. Be present (participate don't anticipate). Roll with it -- allows spontaneous real moments to happen whether on an improv-friendly project or a strictly script-faithful film.
3. Be yourself (be real, not fake). Don't act. Just be.
Often on set, I would try to find a quiet moment to allow the actor to silently connect with that part of the character that is part of their authentic self and use that as a way in. In this way, the actor would only need to be themselves and not try to "act" which usually looks false.