UNAUTHORIZED UNCOORD RWY ENTRY CROSSING.

Narrative

FLT INADVERTENTLY CROSSED THE ACTIVE RWY (I CAN'T RECALL WHICH RWY) WITHOUT ATC CLRNC. THERE WAS AN ACFT ON ABOUT A 2-3 MI FINAL SO GND CTL INSTRUCTED US TO TAXI OFF THE RWY ON THE OTHER SIDE. THE OTHER ACFT DID NOT HAVE TO GAR. THE INCIDENT OCCURRED BECAUSE THE CAPT MISUNDERSTOOD OUR CLRNC AND BECAUSE I HAD MY HEAD IN THE COCKPIT ACCOMPLISHING CHKLIST ITEMS AND THEREFORE FAILED TO BACK HIM UP. ON INITIAL CONTACT WITH GND CTL AFTER PUSHBACK WE WERE TOLD TO TAXI TO THE ACTIVE RWY AND WERE ASKED IF WE WOULD LIKE TO GO AHEAD OF A COMPANY ACFT BEING DELAYED. WE SAID YES AND WERE THEN TOLD TO TAXI TO THE RWY VIA A PERPENDICULAR TAXIWAY LOCATED ABOUT 1000 FT FROM THE APCH END OF THE RWY (SEE DIAGRAM). WE WERE ALSO ASKED IF WE COULD TKOF FROM THAT INTXN OR IF WE NEEDED TO BACK TAXI. WE SAID WE NEEDED TO BACK TAXI AND THE GND CTLR SAID 'ROGER.' THE TAXI DISTANCE WAS VERY SHORT AND I WAS SCRAMBLING TO ACCOMPLISH ALL PRE TKOF CHKLISTS AS THE CAPT APCHED THE RWY. BEFORE I KNEW IT THE GND CTLR WAS TELLING US TO EXIT THE RWY. THE CAPT WAS ALREADY HALF WAY THROUGH A R TURN UP THE RWY (BACK TAXIING). THE CALL SURPRISED BOTH OF US AND HAD TO BE REPEATED ALMOST IMMEDIATELY (ALTHOUGH WITH A CALM; MATTER-OF-FACT VOICE). THIS TIME THE CTLR ADDED THAT WE COULD EXIT STRAIGHT ACROSS; AT WHICH TIME IT HIT THE CAPT AND WE MADE AN ABRUPT L TURN AND EXITED QUICKLY. WE APOLOGIZED TO THE CTLR AND HE ADVISED US THAT THERE WAS 'NO PROBLEM AT THIS TIME.' THE REST OF THE TAXI; TKOF; AND FLT WERE UNEVENTFUL; ALTHOUGH RIDDLED WITH SELF FLAGELLATION; TOTAL DISBELIEF; AND COMPLETE HUMILITY. SUPPLEMENTAL INFO FROM ACN: 203874. WHILE THIS WAS TAKING PLACE WE WERE DOING THE AFTER START; TAXI AND BEFORE TKOF CHKLIST ALSO COORD WITH THE FLT ATTENDANTS FOR DEP ALL THIS OCCURRED WITHIN 1 1/2 - 2 MIN AND IN THE CONFUSION I THOUGHT WE HAD BEEN CLRED TO BACK TAXI ON 23R. THIS WAS A CLASSIC SITUATION OF TRYING TO DO TOO MUCH TOO QUICKLY - TRYING TO BE COOPERATIVE; BUT HURRYING. THE BETTER PART OF VALOR WOULD HAVE BEEN TO GET IN LINE; WAIT THE 5 OR 6 MIN AND THEN PROC! A GOOD LESSON FOR A 20 YR CAPT.