This is from an interview with Joss Whedon in 2006. Be sure to read the interview for the details that go with each piece of advice. I've copied out my favorite one in full below, and it's also worth noting the paradoxical irony of items #6 and #9 - Whedon recommends both listening to what others tell you... and NOT listening! :-)
1. FINISH IT
2. STRUCTURE
3. HAVE SOMETHING TO SAY
4. EVERYBODY HAS A REASON TO LIVE
5. CUT WHAT YOU LOVE
6. LISTEN
7. TRACK THE AUDIENCE MOOD
8. WRITE LIKE A MOVIE
9. DON’T LISTEN
10. DON’T SELL OUT
My favorite is definitely #4, so I've copied that in full here:
4. EVERYBODY HAS A REASON TO LIVE. Everybody has a perspective. Everybody in your scene, including the thug flanking your bad guy, has a reason. They have their own voice, their own identity, their own history. If anyone speaks in such a way that they’re just setting up the next person’s lines, then you don’t get dialogue: you get soundbites. Not everybody has to be funny; not everybody has to be cute; not everybody has to be delightful, and not everybody has to speak, but if you don’t know who everybody is and why they’re there, why they’re feeling what they’re feeling and why they’re doing what they’re doing, then you’re in trouble.Among Joss Whedon's many accomplishments is the genius science fiction series Firefly.
And here's the Firefly cast (image from Wikipedia):
