There's nothing more important. Creative people like to have an image of freedom and lightness, and we all have the reputation of slackers.
But being a writer or actor or director or whatever it is, it takes huge discipline. Especially if you want to succeed.
Projects are easy to start, impossible to complete. That's why the discipline is important. You've got to write the pages, not munch on the cake. You've got to edit the scene, or read the book, not spend three nights googling images of Megan Fox nude.
Tomorrow always feels like a better day to start, whereas today is perfect for one more day of watching 'Friends'. We have valid excuses: the laptop is broke, the girlfriend is sick, the inspiration hasn't struck. But six months go by.
Discipline is staying on track, every day. It's a diet regime that you stick with, for life. It's a gym membership that actually gets used. The good thing is that discipline is the ingredient that makes you an expert, that makes you choose a script over Facebook, or audition preparation over the Xbox 360.
It rarely feels good when you start doing the work. But once you're doing it, you are freed. And it's always easier than you think. You avoid something simple like sending an email to a producer, or enquiring about an agent-- you avoid it for seven months for no good reason, and you feel awful every day. But when you make the phone call or send the email, its easy, and it only took fifty seconds. And your world opens up.
The work is hard to start. Harder to finish. But it's the only way you'll be satisfied come the end of the night.