Legend has it that the boy who played Lady Macbeth died backstage during the play's very first performance. It is also claimed that the ingredients of the famous witches' spell is an actual word-for-word copy of a real spell! Black magic practitioners of the time were not best pleased and they put the hex on the play which has haunted it ever since.
In 1849, three New York theatres were simultaneously putting on performances of the play. One star got friends to disrupt the performance in one of the other theatres. This turned into a riot in which over 30 people were killed. During an Old Vic company tour with the play, among the cast there were an attempted suicide, an electrocution, a gouged eye and an accidental stabbing. In 1947, one actor playing the lead - who had laughed at the supposed curse - was stabbed during a fight scene and subsequently died of his wound; his child died and his widow went mad. In the 1960s, one Stratford actor involved in the play was stabbed and the company manager was strangled. Even as recently as the mid-90s the director of the play in Melbourne - who had also laughed at the curse - lost two of the actors to early deaths.
That's probably enough to explain actors' reluctance to speak the word, but if you click
here a link will take you to a web-page with further information.