A GM also has to be the tactician for the NPCs. There are various ways for GMs to run the opposition in battle.
The opposition can react based on what the players characters do. If the hero brick squares off against the villain mentalist, the villain speedster could intervene. If the hero swordsman prepares to attack the enemy wizard, the enemy archer could attack the swordsman first, or else attack another hero who may be a more dangerous threat.
GMs also need to take on the role of actor. When heroes encounter villains, allies, or neutrals, they want for them to be interesting enough to be able to tell one from another. When the NPCs perform their heroic or dastardly deeds, they should remain feasibly consistent with what the players have already learned about them.
Cult of Personality
NPCs are people too! They need to have personalities. Often, by taking an existing character that the GM knows well, whether its Barney Fife, Ferris Bueller, or his own second cousin, he can use the existing personality for an NPC (without letting the players know about the hidden connection). This will guarantee consistency, as long as the GM keeps straight that Miles Brogan, barroom brawler, is actually Rambo in a different body and an Irish accent. Read more