CAPT OF A B757 FAILED TO FOLLOW TAXI RTE FOR TKOF RESULTING IN ATC INTERVENTION TO CANCEL PREVIOUS TKOF CLRNC AND RERTE RPTR'S ACFT. FO WAS HEADS DOWN ON CHKLIST.

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WHILE TAXIING FOR TKOF (TXWYS B; S; RWY 10R) RWY 10R AT SFO; THE CAPT MISSED THE TURN AT TXWY S1 AND INSTEAD CONTINUED UP TXWY S FOR RWY 10L. I WAS HEAD DOWN COMPLETING THE FINAL ITEMS OF THE BEFORE TKOF CHKLIST WHEN I LOOKED UP AND WONDERED WHY WE HADN'T TURNED ONTO THE RWY AS WE WERE CLRED FOR TKOF RWY 10R. AT THIS POINT; SFO TWR CANCELED OUR TKOF CLRNC AND TOLD US TO HOLD SHORT OF RWY 10L. THE CAPT STILL DIDN'T UNDERSTAND WHAT HAD HAPPENED. I HAD TO EXPLAIN. FORTUNATELY; A SHARP TWR CTLR HAD MORE COMMAND OF THE SIT THAN WE DID. THERE WERE NO TFC CONFLICTS AND WE DID NOT ENCROACH ON OR INTRUDE UPON ANY OTHER RWYS; BUT THE POTENTIAL THERE WAS VERY GREAT. CONTRIBUTING TO THIS NEAR RWY INCURSION (I'M SURE HE WOULD HAVE TAXIED ONTO RWY 10L THINKING IT WAS RWY 10R) WAS THE TIME OF DAY; XX20L; THE END OF A 3 DAY TRIP WITH ALL DAY FLYING WITH THE EXCEPTION OF THIS LEG. FIRST; AFTER FLYING TOGETHER ALL MONTH; I SHOULD HAVE BEEN CLUED IN TO OUR RELAXED STATE OF ALERTNESS AS EACH OF US WERE COMMITTING SOME UNCHARACTERISTIC SMALL ERRORS. SECOND; THE CAPT ONLY FLIES EVERY OTHER MONTH -- SPENDING THE INTERVENING MONTHS IN THE SIMULATOR; AND I FEEL HE WAS NOT AS FAMILIAR WITH THE TAXI RTES AT SFO AS I MIGHT HAVE ASSUMED -- MADE ESPECIALLY UNFAMILIAR AT NIGHT. I SHOULD HAVE ASSISTED MORE WITH GND NAV TO THE RWY. FINALLY; I LOST SITUATIONAL AWARENESS WHEN I CHOSE TO GO COMPLETELY HEAD DOWN TO ACCOMPLISH A CHKLIST; SIMPLY ASSUMING HE'D TURN WHERE HE SHOULD ONTO THE RWY.