The character played by Daniel Hoevels was supposed to commit suicide in the drama with a blunt stage weapon but had instead been provided with a real blade.
He collapsed on stage with blood pouring from his neck and the audience started to applaud the spectacular special effects.
It was only when Hoevels, 30, failed to get up to take a bow at Vienna’s Burgtheater in Austria that they realised something was wrong.
Now police are investigating their own murder mystery drama – after refusing to rule out the possibility that the stunt may have been an attempt to bump off the actor by a jealous rival.
Police have been told that the knife had been bought at a local store and are asking if props staff forgot to blunt the blade for the performance of Friedrich Schiller’s play Mary Stuart, about Mary Queen of Scots.
"The knife even still had the price tag on it," said one shocked police investigator.
The theatre’s props manager is understood to have been quizzed by police about the knife, reported Austrian daily Osterreich.
The actor recovered after emergency treatment to his wound at a local hospital and appeared on stage the next night with a bandage around his neck.
"If Hoevels had hit an artery or cut only slightly deeper, he would have died on stage," a doctor said.