Where you have
items = items.Where(i => actor.Id == id);
Which is just plain wrong, methinks.
What you need to be doing is more like
items = items.Where(i=>i.Actors.Contains(id)
(sorry - that's not valid Linq but you hopefully see what I mean! - not in front of a PC at the mo and Linq not my strong suit!)
So you want the list of movies where that movie's collection of actors includes the actor with the Id you specified...