B757 CREW HAD RWY INCURSION.

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JUST PRIOR TO PUSHBACK MY FLT RECEIVED AN AMENDMENT TO OUR DEP CLRNC. WE PUSHED BACK FROM GATE; STARTED ENGS; AND RECEIVED TAXI INSTRUCTIONS FROM GND TO TAXI TO RWY 28L BUT HOLD SHORT OF RWY 1L. I READ BACK AND UNDERSTOOD INSTRUCTIONS AND ERRONEOUSLY ASSUMED CAPT UNDERSTOOD THEM AS WELL. WHILE TAXIING; MY HEAD WAS DOWN MOMENTARILY AS I WAS BUSY INSERTING THE FINAL LOAD NUMBERS; SPDS; AND UPDATING OUR AMENDED DEP CLRNC INTO FMC. MEANWHILE; CAPT HAD MISTAKENLY TAXIED ACFT ACROSS RWY 1L TO HOLD SHORT OF RWY 1R. GND CTL CALLED TO INFORM US OF OUR MISTAKE. I THINK THE FOLLOWING CORRECTIVE ACTIONS COULD BE TAKEN TO PREVENT THIS TYPE OF MISCOM IN THE FUTURE: 1) AS FO; I NEED TO HAVE A HEIGHTENED AWARENESS OF RWYS WE ARE XING OR HOLDING SHORT OF NOW THAT I'M ON A FMC ACFT WHICH OCCASIONALLY REQUIRES HEADS DOWN UPDATES. 2) AS FO; I SHOULD HAVE BEEN MORE POSITIVE THAT THE CAPT FULLY UNDERSTOOD OUR TAXI INSTRUCTIONS. 3) THE CAPT SHOULD HAVE READ BACK THE HOLD SHORT INSTRUCTIONS SO THAT WE BOTH WERE IN AGREEMENT. 4) THE CAPT SHOULD HAVE ASKED ME IF WE WERE 'CLR ON THE R?' PRIOR TO XING RWY 1R. THIS WOULD HAVE GIVEN US ONE LAST CHANCE TO CATCH THE MISTAKE. 5) GND CTL MIGHT ALLEVIATE SOME CONFUSION BY SIMPLIFYING TAXI INSTRUCTIONS TO ONLY THE POINT ACFT IS CLRED TO HOLD SHORT OF. POSSIBLY; IN RETROSPECT; THE PRUDENT ACTION WOULD HAVE BEEN TO UPDATE OUR DEP ROUTING IN THE FMC PRIOR TO TAXIING AT ALL. ALTHOUGH I DON'T BELIEVE A CONFLICT AROSE FROM OUR PARTICULAR RWY INCURSION; ONE COULD HAVE AND I THINK IT'S IMPORTANT FOR ACRS TO NOT BE COMPLACENT ON GND (ESPECIALLY IN FMC ACFT WHERE ONLY 1 PLT MIGHT BE MONITORING ACFT'S PROGRESS). ALL GND TAXIING CLRNCS NEED TO BE TREATED WITH AS MUCH RESPECT AND POSITIVE; PROFESSIONAL AND DELIBERATE COCKPIT COMS AS INFLT CLRNCS.