Here are some bits of advice that master screenwriters had to say about their craft.
As a young journalist I thought that stories were simply what happened. As a screenwriter I realized that we create stories by imposing narrative on the events that happen around us.-
Nora Ephron (When Harry Met Sally, Sleepless In Seattle)
I consider my job as a screenwriter to pack a script with possibilities and ideas - to create a feast for the filmmaker to pick from.
Jonathan Nolan (Interstellar, The Dark Knight Rises)
What’s the trick to writing a genuinely funny comedy? The trick is therapy. Take notes.
Lake Bell (In a World…)
There is no point in having sharp images when you’ve fuzzy ideas.
Jean Luc Godard (Breathless, Contempt)
Make it dark, make it grim, make it tough, but then, for the love of God, tell a joke.
Joss Whedon (Cabin in the Woods, The Avengers)
I try to write 'and it's all very funny' after each scene description so that the reader can imagine the movie in their head.
Noah Baumbach (The Squid and the Whale, Frances Ha)
There is no terror in the bang, only in the anticipation of it.
Alfred Hitchcock (Psycho, Rear Window)
It's more interesting for me as an audience member to see a movie about a loser.
Ethan Coen (The Big Lebowski, Oh Brother Where Art Thou)
All good ideas start out as bad ideas, that why it takes so long.
Steven Spielberg (Poltergeist, Close Encounters of the Third Kind)
Write out the scene the way you hear it in your head. Then read it and find the parts where the characters are saying exactly what you want/need them to say for the sake of narrative clarity (e.g., “I’ve secretly loved you all along, but I’ve been too afraid to tell you”). Cut that part out. See what’s left. You’re probably close.
Andrew Bujalski (Computer Chess, Beeswax)
You have to read widely, constantly refining (and redefining) your own work as you do so. If you don’t have time to read, you don’t have the time (or the tools) to write.
Stephen King (Pet Sematary, It)
A tip from Lubitsch: Let the audience add up two plus two. They'll love you forever.
Billy Wilder (The Apartment, Some Like It Hot)