Frank T. answered 04/30/19
Ph.D. in Film & TV Studies (UCLA) with 35 years of Teaching/Publishing
Often this is just done with a knife or sword with a retractable blade, a prop often used in the theater. A "blood squib" can be added so that it looks like blood is flowing from the wound. Add some dramatic music, and most viewers will be distracted enough to not pay attention.
Nowadays, CGI effects can also be used to create the illusion of stabbing but that's usually very expensive, unless something very special is required (i.e., the dagger has to go all the way through someone's neck or chicken noodle soup has to burst from the wound instead of blood. :-)