Well, yes, in that case it was, "I'm compelling an aspect you don't know about. If you take the fate point, you're going to be attacked by ghouls." Obviously they're going to get the immediate result of the compel told to them so they can make a decision on whether to take it (though I've got at least one player who pretty much always takes the compel before I even have a chance to tell him what it is). It's just that sometimes telling them the aspect (in this case, the NPC's aspect was along the lines of "THE VILLAIN OF THE WHOLE PLOT") gives too much away too soon.