AN ACR B757 FLC WAS GIVEN DETAILED INSTRUCTIONS FOR TAXI TO RWY 4L. AT THE END OF THE INSTRUCTIONS THEY WERE; APPARENTLY; ALSO TOLD TO FOLLOW ANOTHER ACR. UNFORTUNATELY; THE OTHER ACR WAS GOING TO RWY 9L. THE ATCT LCL CTLR ALLOWED THEM TO DEPART ON RWY 9L AFTER THEY MISTAKENLY TAXI INTO POS ON IT.

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DURING INITIAL TAXI OUT; GND TOLD US TO GO TO RWY 4L; FOLLOW ANOTHER ACR FROM OUR L. WE FOLLOWED HIM OUT; SWITCHED TO TWR PER INSTRUCTIONS EARLY IN THE TAXI. UNFORTUNATELY; I FOLLOWED THE OTHER ACR TOO MUCH. HE WENT TO RWY 9L. TWR CLRED US INTO POS ON RWY 4L(?) (HE MUST NOT HAVE BEEN WATCHING; OR WE MISUNDERSTOOD HIM.) AS WE ENTERED THE RWY; I NOTICED THAT THE RWY SYMBOL ON OUR FMC MAP DID NOT AGREE WITH THE DIRECTION OF THE CONCRETE; AND SAW '9L' PAINTED ON THE RWY. TWR CLRED US TO TKOF ON RWY 9L. THE FO HAD THE WTS IN MIND AND SAID WE WERE OK ON WT; NO TFC CONFLICT; SO WE TOOK OFF. REMEDY FOR NEXT TIME: 1) DON'T MINDLESSLY FOLLOW ACFT ON TXWY. 2) LOOK AT ELECTRONIC MAP BEFORE TAKING POS. 3) STAY ALERT. SUPPLEMENTAL INFO FROM ACN 377061: WE GOT A LOT OF INSTRUCTIONS VERY EARLY (GND WAS SLOW); BUT THIS SHOULD NOT NORMALLY BE A PROB. WE HAD A DISTRACTING DISCUSSION WITH THE GND CTLR; BUT THAT HAD ENDED LONG BEFORE WE MISSED OUR TURN. I HAD MY MAP OUT (ALWAYS DO); BUT SOMEHOW; IT DIDN'T 'CLICK' THAT WE NEEDED TO TURN ON 'W' FOR RWY 4L. I HAD BEEN FLYING WITH THE CAPT ALL MONTH. A VERY COMPETENT PLT WITH A METICULOUS ATTN TO DETAIL. I DON'T FEEL I HAD TO 'WATCH HIM.' WE WERE TOLD TO 'FOLLOW THE OTHER ACR.' PERHAPS A BETTER TERM WOULD HAVE BEEN 'GIVE WAY TO.' PERHAPS I WAS COMPLACENT. PERHAPS I'M NOT BACK ON TOP OF MY GAME (I'D JUST RETURNED FROM A 14 MONTH LEAVE OF ABSENCE). I'M JUST SEARCHING IN HINDSIGHT FOR AN ANSWER TO 'WHY?' I THINK A COMBINATION OF ALL OF THE ABOVE. I THINK THE MOST DISCONCERTING PART OF ALL THIS FOR ME IS THAT I DIDN'T REALIZE THE ERROR WAS MADE UNTIL IT WAS MUCH TOO LATE -- AND THAT I CAN'T COME UP WITH 1 GOOD REASON AS TO HOW OR WHY IT HAPPENED.