An assistant behaves obstreperously during the act and the performer's temper being sufficiently strained, s/he condemns the assistant to death by decapitation. The assistant is restrained face-downwards upon a large oblong box — perhaps with just a suggestion of a coffin in shape — and his/her head is covered with a cloth. The performer wields a sufficiently large knife and saws off the assistant's head; s/he plucks it up by the hair and waves it around and walks with it here and there on the stage, gleefully proclaiming insults. S/he covers the head again (to sop up the blood, perhaps?), places it at the other end of the box, at the feet of the body; removes the cloth again and mockingly places a lighted cigarette in its mouth — after all, the head is dead, so what harm can a cigarette do to it? But the head suddenly begins to puff on the cigarette, spits it out and demands to know that the performer has done to it! Aghast, the performer covers the head again, places it at the neck of the body, kneads the neck a bit, and the assistant suddenly stands up and jeers at the performer!
This is rather a resource-intensive illusion, requiring two assistants and a model head. Ideally, the assistants would be identical twins but failing that they must be very similar in build and appearance and must be dressed and made up identically. The model head must also be identical to the assistants' heads.
The box has two holes concealed by drapery or upholstery or something; one is at the "head" end and the other by the feet of the first assistant. When the first assistant is laid upon the box, as soon as his/her head is covered by the cloth, s/he ducks his/her head into the hole and the other assistant pushes the dummy head out. The performer places the dummy head over the second hole, whereupon the second assistant pulls it in and pokes his/her head out. And, of course, the procedure is reversed to complete the illusion.
For maximum effect the model head can have dripping blood controlled by a valve somewhere on its surface that the performer can turn on and off. The first assistant must breathe very imperceptibly although the performer can make sure that enough is taking place to constitute misdirection; additionally s/he'll be standing in front of the body while placing & removing the dummy head.
The original description of this trick had the performers dressed in garish clown costumes, and the executioner was dressed as a harlequin. It's doubtful these days that anybody knows who harlequin is or what he looks like. Even so, the executioner could perfectly well be another assistant, menacingly dressed for his gruesome role.