That particular insight was mined from James Scott Bell's The Art of War for Writers, a fantastic book to have just digested considering my busy November. Another great I finished only last week is Donald Maass' Writing the Breakout Novel. If you haven't read these both, I highly recommend them.
To be clear, a synopsis is second cousin to an outline or treatment (although I believe the term treatment is more commonly used for screenplays). A synopsis' purpose is to detail all the differing plot lines, scenes, etc. In other words, it's a semi-detailed recipe of what happens.
A premise is more basic and simply sketches the broad outlines of the story in general terms, with enough detail in a few lines to catch someone's interest. This is also similar in scope to a pitch. If a synopsis is a photograph, a premise is a watercolor painting.
My preference in a premise is to try to capture three primary components:
1) The chief story problem/inciting incident.
2) The action/reaction of the main character to the story problem or situation. This sustained motivation and the obstacles encountered become the general confict arc of the novel.
3) Complications or reversals needed to sustain the second half of the story, and keep up conflict.
What I discovered over the last year--and this seems more true of story ideas I am confused about or unsure of--is that the act of getting the premise down on paper really focuses things. I am forced to choose specific actions through careful verb choice, pick main characters, untangle confusing scenes or deal with unrealistic motivations. Nine times out of ten, I walk away from the premise-writing exercise with a much clearer understanding of the story I am trying to write or edit.
At this early stage, this is also true of my NaNoWRiMo project, Daisy. Here it is for you reading pleasure:
Doctors diagnose Daisy who's been in and out of hospitals for most of her sixteen years with a rare and lethal form of brain cancer. Opposed to an experimental treatment that promises to turn Daisy’s final days into a torturous, excruciating nightmare, her father Kodi “kidnaps” her.
Together father and daughter flee across the country, trying to outwit law enforcement and stay one step ahead of Daisy’s mother, who has made their flight—and the uncertain future of her daughter—a national news story. On the road, Daisy and Kodi each discover a love they never bargained for, and learn that life is about more than looking for an exit.
I'd be very interested in hearing your comments or suggestions. It will be fascinating to see if this premise is still accurate after the first draft is done. I'll keep you posted.
What about you? What process(es) do you use to refine your understanding of you WIP? Got a premise you'd like to share in the comments?