I remember an actor from Gorky Park, Alexei Sayle, telling how in one scene he opens a door to be shot in the head. The thing was he was told not to react to the shot, just to fall over. However they were using some form of 'mild' squib which was some how camouflaged. So every time they tried it, the shock of the squib going off caused him to react wildly (as you would).
In the end he said they just got a drinking straw and packed the end with a 'blood' soaked pulp then someone blew it at him at the required time.
I know the scene he was talking about and it looks very effective, if I remember right.
Steven