COMMUTER ACFT TAXIES ONTO ACTIVE RWY.

Narrative

AT THE TIME OF THE RWY INCURSION BOTH THE CAPT AND I WERE FATIGUED. WE WERE AT THE END OF A SEQUENCE OF 'DAY TRIPS' ORIGINATING AT DCA. TODAY'S DUTY DAY WAS SCHEDULED FOR 14 HRS -- NOT INCLUDING THE TIME REQUIRED TO DRIVE/COMMUTE TO THIS BUSY METROPOLITAN ARPT. THE CAPT HAD FLOWN 1 MORE DAY THAN I HAD AND WAS VISIBLY FATIGUED FROM OUR MORNING CHK-IN UNTIL THE TIME OF THE RWY INCURSION. HE WAS MISSING VIRTUALLY EVERY RADIO CALL WHILE HE WAS PNF. THE CONFUSION LEVEL WAS VERY HIGH AS COMPANY ACFT AND WE TAXIED OUT AT THE SAME TIME FROM OUR RAMP AT JFK. COMPANY WAS SUPPOSED TO FOLLOW US BUT DUE TO RAMP CONGESTION THEY ENDED UP IN FRONT OF US. COMPANY TOLD GND (AND WAS ACKNOWLEDGED -- I ALSO INFORMED GND THAT WE WERE BEHIND COMPANY BUT RECEIVED NO REPLY). GND INSTRUCTED MY ACFT TO CROSS RWY 31L AT K AND PROCEED TO RWY 4L FOR DEP. COMPANY (WHO WAS ACTUALLY IN FRONT OF US; WAS TOLD TO HOLD POS; WHICH WAS ON THE OUTER AND KK). I AGAIN TOLD GND THAT WE COULD NOT PROCEED BECAUSE COMPANY AHEAD WAS BLOCKING US; BUT RECEIVED NO REPLY. NEXT; THINGS HAPPENED VERY QUICKLY. COMPANY WAS TOLD TO GO INTO POS ON RWY 31L FROM KK FOR DEP; HENCE THE RWY BECAME ACTIVE EVEN THOUGH GND CTL HAD CLRED US TO CROSS; ALBEIT UPWIND; BUT WE HAD NOT DONE SO DUE TO BEING BLOCKED. MY CAPT SAID 'I'M GOING TO FOLLOW HIM.' (HE WAS DISORIENTED AND NOT AWARE OF WHAT WAS GOING ON.) I SAID 'THAT'S NOT WHAT HE (GND CTLR) WANTS US TO DO!' IT WAS TOO LATE; AS WE CROSSED THE HOLD SHORT LINE AND GND CTL TOLD US TO 'HOLD OUR POS!' COMPANY THEN DEPARTED UNEVENTFULLY AND WE WERE CLRED TO CONTINUE OUR TAXI TO RWY 4L. CONCLUSION: PLT FATIGUE -- MY CAPT; ALWAYS VERY SHARP AND CONSCIENTIOUS; WAS FATIGUED AND IT SHOWED. HIS LEVEL OF AWARENESS WAS NOT UP TO PAR. MY FAULT; THROUGH IMPROPER COCKPIT RESOURCE MGMNT COULD NOT COMMUNICATE OUR SIT TO THE CAPT EFFECTIVELY ENOUGH TO GET HIM TO STOP THE ACFT. REASON: I WAS FATIGUED ALSO.