Performance Technique
Natrualism
Coming from the Theatre Libre and other traditions of french naturalism, the early Grand-Guignol was characterized with radical notions that blurred the line between the stage and real life.
Chief among these radical ideas were "realistic" portrayals of the working class. That, coupled with staging that completely ignored the audience (so much so that entire scenes could be delivered upstage), lead to the moments of gore and horror being perceived as all the more "realistic".
Melodrama
Springing from the Fait Divers often exaggerated true crime narratives, Melodrama also had its home on the Guignol stage. The flair for the melodramatic can most closely be seen in the villians of shows. Ranging from Henri's portrayal in Le Baiser dans la nuit to the popularization of the mad scientist archetype, these characters were flatly melodramatic with their villainy (often verging on moustache twirling).
Expressionism
Considering adaptations of The Cabinet of Dr. Caligari graced the Grand-Guignol stage, it should be no surprise that expressionism creeped its way into the actors performances. The visuals of the Grand-Guignol also leant themselves more to expressionism during the theatre's run. Most things, from Makeup to lighting and set design all borrowed heavily from the then contemporary german expressionist movement to shock their audiences.
The Actor or The Magician?
An oft ignored performance aspect of the Grand-Guignol is the delicate balance the actor must have made between the outward emotional performance of acting and the hidden calculated performance of stage magic required by many of these plays.
Blood could be hidden in props, costumes, or on the set. Fake eyeballs and limbs had to be perceived as real as they were mutilated. Trick weapons and stage combat were required by the text.
So in what ways were these actors, what we now conceive of as 'actors'? And in what ways were they stage magicians creating a horrifying act?