Start with the outcome, not the deliverable
"We need a 60-second film" is a deliverable. "We need to convince mid-career engineers that we are a serious employer" is an outcome. The second one gives us something to work with.
Tell us what you don't want
References of work you hate are more useful than references of work you love. They tell us where the boundaries are.
Trust the people you hired
If you have picked the right agency, the best results come from briefing the problem and getting out of the way.