DAVINA: The English monarchy of Shakespeare’s era was a relatively stable political system. When a monarch died, the rules of succession determined who was next in line for the throne so that the transition of power generally went smoothly.
RALPH: The same could not be said of 11th-century Scotland, where, for centuries, several powerful clans had competed for supremacy, and there was the constant threat of invasion from England to the south or marauding Vikings from across the North Sea.
DAVINA: In this war-like environment, it was crucial that the king be a successful military leader. Therefore, instead of the crown passing automatically to the king’s eldest son, the nobles selected their best warrior to be king.
RALPH: Since Macbeth appears to be the best military leader in the realm, it seems quite plausible that he could become king without having to take any drastic action.
DAVINA: But Ralph, given what you’ve just said, isn’t it odd that the king is miles away from the battlefield while his generals, Macbeth and Banquo, are fighting such important battles? 
RALPH: That’s a very good point, Davina.