It depends on which character it is, and what it is they wish for. If it's something small, they tend to get it - or at least something like it. If it's something big, on the other hand ("I want this war to end", "I wish someone could love me", "I wish I could do [x]", etc.), they might get it, but they'll have to work for it. No such thing as a free lunch, and all that.
Of course, character's wishes tends to change over the course of the plot. If they start out wanting to kill with [x]-person, they might end up not wanting to kill them later, because they've learned more. Or the other way around.