Mix up a batch of red food dye/corn syrup and add to it a fair amount of dark vinegar and put it in a cup. Then get the actor to sprinkle bicarbonate of soda/baking powder in his/her mouth. When you're ready to take the shot have the actor drink the blood/vinegar mixture and hold it is their mouth. Start the camera rolling because quickly the vinegar and bicarboate of soda are going to react and start frothing and expanding.
Make sure they don't swallow the mixture, because it's probably kind of gross.