If you want the snarky answer...
Most beginnings of most deals are made over the telephone and over cocktails, they get fleshed out at lunches and parties. The complete deal is put together over a very nice dinner. Then it is handed over to attorneys to be put in legal speak.
This is a part of the social aspect Rik is talking about.
You spend a huge portion of your life being seen in all the right places with all the right people.
Your network isn't necessarily deep, but very broad.
You keep a very, very detailed tickle file (or whatever they call it these days) on everyone you meet, remember to send birthday and anniversary greetings, ask about the wife/kids/whatevers when you see them.
You go to screenings, film festivals, launch parties, afters parties, seminars, meet & greets, etc., etc., etc. constantly - you are almost never at home.
You need an encyclopedic memory for people.
You need to be noticeable and memorable without being a (pick your favorite expletive).
You ALWAYS need to follow up on what you say you will, even if it's just to say no.
You NEVER burn any bridges (todays junior a$$hole, tomorrows executive producer... Or perhaps Big Names brother-in-law).
You never promise anything you can't deliver.
Treat the worker bees with respect.
I think you get the idea.