You have to get inside the clients head, they usually say one thing and mean another (not always of course).
At the brief (as said above), try and get a musical directions.
It's always do-able. But sometimes another composer my come up with a better idea that's do-able.
If it's a cover or rehash, do they want the same tune in another style from another period at another tempo?
Orchestral, EDM, Rock etc.
Questions, ask questions, and be very polite about it.