Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Human Factors
Captain flying descent on BUFIE arrival. Captain reset altitude window for descent on arrival to lowest crossing limit. Captain commanded aircraft start down early for crossing restriction with the expectation of LNAV clearance direct to SLI at our request. Aircraft began descent for crossing restriction and when I noticed altitude deviation I told the Captain. Captain immediately corrected the deviation. ATC issued a clearance direct SLI immediately after the deviation.
When the Captain of a B757-200 preset the bottom altitude of an anticipated RNAV STAR clearance he failed to ensure an appropriate VNAV mode and the aircraft began a premature descent.  The First Officer intervened and ATC shortly issued the anticipated clearance.