Are you using 8-directional behavior for the NPC movement?
If so,
Did you check that 'set angle' is set in the 8-directional properties?
If you are moving it via events (e.g. lerp), you just need to do this:
every tick:
instance variable angleOfMovement = angle(previousX, previousY, self.X, self.Y),
previousX = self.X
previousY = self.Y