Writing the pilot is the toughest part of writing television. You're writing an episode, creating a template, and setting the tone for the show. You're creating the voices for the characters. You're teasing the audience, giving them a sense of what the show's going to be like.
You're creating the show. And, oh, you don't have a writing staff yet. And if you get it wrong, you won't get a writing staff, or another episode to make it right. This is it, baby. This is where you prove that you really have a good concept -- or not.
What is the hook? The pilot has to sell that.
What is the attractive fantasy
-- what is the wish fulfillment your show gives the audience, if any. (Not all shows have it. Oz
has a negative fantasy; so do most comedies.)
Who's the central character?
What happens every week?
Who's core cast and who's just recurring?
By the time I get to writing a pilot, I've already written what amounts to a show bible. That's just me; other writers with more impressive credits don't do that. (It's also the Canadian TV system, which seems more oriented to written pitch bibles than the US does.) I've got a dozen pages of sizzle about the show, descriptions of characters and possible springboards for episodes.
The moment you start writing the pilot, you start realizing that some of the stuff in the pilot won't work. The character you might have thought was central may not work. Another character may come alive and insist on more attention. You have to trust yourself. No one ever won a battle without a battle plan -- but smart generals are ready to change their plans when obstacles or opportunities demand it. Same with writing a pilot...