Mix one tablespoon of water with one cup of clear corn syrup in a bowl. Add drops of red food coloring, and stir the mixture with a whisk.
Stir in a small bit of chocolate syrup, using a whisk, to give this fake blood the desired, realistic consistency. Allow the mixture to sit for about 10 minutes.
Place one video camera in a position to capture one actor's stabbing motion towards the victim.
Position a second video camera on the opposite side of the victim, so that the victim's body blocks the camera from actually seeing the knife "entering" the victim's flesh.
Film the action. One actor stabs towards the victim with a retractable plastic knife, caught by the first camera. With the second camera, capture the victim's facial reaction as he is stabbed, which may include spitting out the edible fake blood.
Smear the fake blood onto the victim's "stab wound." Film the wound and the aftermath of the stabbing.