At one time I thought the most important thing was talent. I think now that the young man or the young woman must possess or teach himself, training himself, in infinite patience, which is to try and to try until it comes right. He must train himself in ruthless intolerance--that is to throw away anything that is false no matter how much he might love that page or that paragraph. The most important thing is insight, that is to be--curiosity--to wonder, to mull, and to muse why it is that man does what he does, and if you have that, then I don't think the talent makes much difference, whether you've got it or not.
I recently participated in a free teleconference put on by the ISA with the legend himself, Robert McKee. Robert had a proliferation of valuable advice to dispense over the hour-long Q&A session, and I did my best to take notes on what I found to be his most interesting points.
This great article by Jeanne Veillette Bowerman talks about her personal experience with balancing having the necessary patience to carefully peice together your story and the productivity to start writing the damn thing already.