As a GM, when you're running a game, there is a lot of creative pressure on you. NPCs need names, personalities, and all that stuff to be 3 dimensional. Cities need to come to life, countries need to work. It can really start to bog you down if you don't go in prepared, especially since you never know when your players are going to choose to burrow a tunnel under the city, when the choice before them was "Do we want Chinese or Pizza for dinner tonight?". Of all those things though, the hardest for me is making NPCs on the fly. I assume, unless making characters is your most favoritest thing ever, that it is similarly hard for you as well.
The thing is though, there are tricks you can use to help you along with this, especially for important NPCs. Odds are you've been playing RPGs for a few years and have made about a half dozen or more characters for yourself to play. So why not use those characters again? Base an NPC off their personality, and dust off an old favorite to be a big NPC in the new game. You'll need to change the character to fit in, but the personality can generally be straight lifted out for your needs. While you're doing that, what about characters in books you've read? Or the characters other people played?
Without knowing it, or maybe with knowing, you've been exposed to dozens upon dozens of characters, most of whom are probably fairly solidly built and defined in your head from how you were exposed to them. So why not lift what you need out of those people and use it again? You need a King? Lift some of the leadership qualities and flaws from a couple other characters, mix them together and voila instant King. The same holds true for almost any other type of character as well. They're there, in your head, so why not put them to work to earn some rent?
If you are going to straight borrow someone else's character you should ask them, but odds are they'll be fine with it, even flattered. Odds are you won't need a straight lift either, since what you are after is personality more than anything else, that other stuff like race, gender, job, etc are probably defined by the story's needs. So, have fun with it, and use what you know.