"Fai la guardia."
It doesn't mean the same thing, but you can say 'you be the guard'. If you were playing a game, you could say, "You be the guard and I'll be the prisoner"' for example. I admit that it is a very specific instance, but it is certainly idiomatic English. Having said that, I'm not at all sure that that is what the Italian sentence is saying.:)
Wikipedia: http://en.wikipedia.org/wiki/Imperative_mood#Formation The imperative form is understood as being in the second person (the subject pronoun you is usually omitted, although it can be included for emphasis),
No idea. Oxford English Italian dictionary says that policeman is the second meaning for guardia.
I did the same thing first time around, but I think that the general rule we should follow is to take the first hint they give us, unless we know it can't possibly be right. It makes it easier to decide. If I remember, guard was the top definition.