Yesterday eve, I've watched the director cut of the 1984 movie Amadeus, directed by Milos Forman. What really caught my attention is how the movie introduced Mozart. We all know who he is, and yet, the movie takes 5 minutes to utter his name, 8 to show his back, 10 to show his legs and 15 to show his face. In the meantime, we hear a lot about his greatness, the movie creates a lot of anticipation, and when we finally see him, he's so far from our expectations, that he just disgusted me.
I'm not here to talk that much about the movie, though, but about what we can learn from this. And the truth is, that the effectiveness of this way of introducing is just overwhelming. To make long things short, what I'm here to talk about, is a way to introduce characters, mainly NPCs, in the game.
The way is quiet simple: 3 sessions before the PCs will encounter the NPC, they'll hear rumors about him/her, about the greatness of this particular NPC, and they'll see him/her near the end of the session ,surrounded by admirers, from afar. They won't see the little details, but they'll know it is the NPC.
Two sessions before the encounter, they'll continue to hear about the NPC, and they'll see one of his/her actions. May be he'll save a little girl, or she may save a little boy. May be they'll see him killing a dog, or her killing a lion... Again, all of this from afar, although a little bit closer this time.
The last session before, they won't see the character. The townsfolk will say that s/he is rehearsing for something, or that s/he was called by the emperor to gain a reward.
When the characters finally meet the NPC, they'll be eager to meet him/her, to learn from him, and to talk with her. In this stage, you the GM must make the character as humane as one can, as not only what is commonly known, may be even a little far from that, if one sees fit.
Of course, all that is written is based around the belief that this character is a major character for the plot or for the kingdom or whatever. If the characters is only a background character with no real importance, why spend so much effort on this character?