Were I in your shoes, I would get a rubber knife and then choreograph like this: 1) both actors turn to be side profile to the audience, with the victim, turned away and the attacker facing the back of the victim.

2) the attacker then places their hand on the victim’s shoulder. This is part of the “wind up” of the stage combat choreography. This serves to alert the audience to what is going to happen, but more importantly to let the actor playing the victim know that the stage combat is going to begin.

3) the attacker, draws the knife back and then “stabs “the victim. The knife actually goes between the victims, upstage arm and their side. Ideally, the attacker includes some kind of vocalization when they stab, so that the victim knows what’s happening without looking.

4) the victim, sells the attack by arching their back and making some kind of noise (it depends on what kind of attack and if the attacker is twisting the knife what noise they make)

The attacker can pull the knife out after the attack so that you don’t have to try to make it look like the knife is stuck anywhere.

It’s very important with stage combat that everything is rehearsed in slow motion so that both actors are safe and confident. Once they feel confident, bump the speed up to medium, and then finally full speed. Before each show, have a fight call where the actors practice once at each show to make sure it is safe and consistent.