Your question is too general - there being various states of flight the 777 can be in during descent. Altitude can be commanded and speeds flown to speed and alt constraints in the FMC, FMC speed can be flown by air braking to fly a steeper descent, etc.
Other than speed and alt constraints the FMC numbers are a guide. ATC will, most of the time be issuing speed and altitude commands at you anyway.
Were one to leave the aircraft completely alone to fly the FMC invariably it would try and reach the FMC speed on arrival at the constraint and then be unable to descend at the necessary FPM in order to reach the next desired altitude at the required speed, etc.
During descent the pilot becomes once more the most efficient computer on the aircraft and disposes gradually of the automation systems as control is taken of the flying.