The brief is everything. Your team are not psychic, so if you have a vision of what you want then its essential to develop a brief. The brief can be a guideline, it gives purpose, integrity and value to the assets and projects you create.
With bigger projects, you may want to do a workshop first, this gives an opportunity for everyone to contribute to the brief and if it is well managed, your team will feel more inclined to be open and share.
When writing a brief, start backwards: What is the outcome you want from the project? What do the experts say works best? Research is essential (eg: Facebook adverts; look at current data and thinking around what works for these audiences before creating your video trailer brief).
The briefing can be a collaboration or one person’s vision, but without a brief, things could go pear-shaped, and you will find you have spent money on an inadequate return.