AN A319 CREW; TAXIING FOR TKOF AT SEA; TAXIED PAST A HOLD SHORT LINE.

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I WAS FO ON AIRLINE FLT XXXX FROM SEA TO DEN ON DEC/XA/01. CAPT WAS TAXIING FROM N SATELLITE TO RWY 16L AT SEA. GND GAVE CLRNC OF 'TAXI TO RWY 16L VIA TXWY B.' I ASSUMED SHE KNEW WHERE SHE WAS GOING AND WENT HEADS DOWN FOR APPROX 5-10 SECONDS. WHEN I LOOKED UP WE HAD CROSSED HOLD LINE FOR RWY 16L AT INTXN L. ANOTHER ACFT WAS JUST BEGINNING TKOF ROLL. WE BOTH REALIZED WHAT WAS GOING ON; STEPPED ON BRAKES AND SWUNG ACFT AROUND BEFORE WE REACHED THE RWY PROPER. APPROX 1/2 OF ACFT CROSSED HOLD LINE BEFORE WE TURNED AROUND. ACFT ON TKOF ROLL CONTINUED TKOF AND WE TAXIED TO RWY 16L VIA TXWY B. ATC WAS INFORMED AFTER THE FACT. I HAD BEEN DOING SAME TRIP ALL MONTH AND WAS VERY COMFORTABLE WITH ROUTING TO RWY. THIS IS RELATIVELY SIMPLE RTE AND DIDN'T EXPECT CAPT TO HAVE A PROB. I TAKE RESPONSIBILITY FOR MAYBE BEING SOMEWHAT COMPLACENT AND NOT BACKING CAPT UP AS BEST AS I COULD. LESS HEADS DOWN TIME AND MAYBE WAITING UNTIL ON PARALLEL TO DO COCKPIT CHORES COULD HAVE HELPED AVOID SIT IN FIRST PLACE.