In the first page of The Social Network screenplay by Aaron Sorkin, we first hear the conversation in the bar before moving into the bar and meeting the two characters talking. This is how the protagonist (if not necessarily hero) Mark Zuckerberg is introduced:
INT. CAMPUS BAR - NIGHT
MARK ZUCKERBERG is a sweet looking 19 year old whose lack of any physically intimidating attributes masks a very complicated and dangerous anger. He has trouble making eye contact and sometimes it's hard to tell if he's talking to you or to himself.
This description not only shows you how well cast Jesse Eisenberg was for the part but how adept Sorkin is at quickly--but not in any great rush either--describing the character so everything that comes after is much easier to picture.
In the subsequent scene soon after, which is a flash forward to 3 years later, here's how Sorkin re-introduces Zuckerberg:
INT. DEPOSITION ROOM - DAY
It's three years later and MARK is sitting with his LAWYERS at a large conference table.
Mark is wearing a hoodie, sweatpants and Adidas flip-flops--a personal uniform that we'll come to understand. And while it may take awhile to notice it, Mark's a different person in these flash-forward scenes. Still tortured and complicated, but comfortable now with his own power.
Just a few short lines that quickly sum up how Mark's character has changed in that time span, especially important since this scene occurs in the film right after the first college scenes.
Since we are all taught to be quick in our descriptions, not wanting to make anything too wordy for a reader, sometimes we err too much to the extreme of extremely vague and rushed character introductions. Here's an example of how to do it right, from one of the best.