The creative brief is like an ad written for the creative team
To aid you in crystallizing your task as someone who writes the creative brief—whether you are the advertiser initiating the project with your in-house creative department, an outside creative team or your advertising agency; or you are an agency account person—here's a great analogy:
The creative brief is a print ad that offers the first creative thinking for the copywriter and the art director.
"I once heard a planner ask John Hegarty, the creative director of the top London agency Bartle Bogle Hegarty, what he looked for in a creative brief. He replied that he looked for a very simple, single-minded idea, which is usually expressed in the part of the brief that many agencies term the proposition. Hegarty said that it was his habit to take that one sentence and write it on a large piece of paper, above or below a picture of the product, almost as if the line from the brief were a headline. Then he would pin it up above his desk and ask himself first whether the juxtaposition of that line and that product made some rational sense, and second, whether it also started to suggest something interesting on an emotional level. If so, then he would think, 'There's the first ad in the campaign. It's my job to create something better.'"
"If the writer of the brief finds it impossible to manifest his or her own thinking in an advertising idea, then it will likely be an uphill struggle for the team assigned to create the actual campaign.
"A brief tends to succeed in direct proportion to the level of creativity present in both its idea and presentation. If the creative brief is not itself creative, if it does not suggest solutions to problems, present information in an expansive and interesting way, and interpret that information with imagination and flair, then its authors and presenters have no right to expect anything different from their creative teams." (emphasis added)