A B717-200 CREW; TAXIING FOR TKOF AT MIA; TAXIED ONTO AN ACTIVE RWY WITHOUT A CLRNC.

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THE CAPT BEGAN HIS TAXI FROM SPOT X TOWARDS RWY 12. WE WERE THEN TOLD TO CHANGE OUR DEP RWY TO RWY 9L; 'TAXI TO RWY 9L VIA TXWY P.' I BRIEFED THE CAPT ON THE TAXI RTE; 'CONTINUE AHEAD TO TXWY P AND TURN R; THAT IS THIS SIDE OF RWY 12 AND THAT TAKES US TO THE END.' HE HAD HEARD ATC; MY INSTRUCTION; AND HAD HIS DIAGRAM OUT. HIS RESPONSE OF 'UHUH' AND HIS FAMILIARITY OF THE ARPT MADE ME THINK HE UNDERSTOOD. I WENT HEADS DOWN TO REPROGRAM THE FMS TO RWY 9L AND PREPARE FOR THE NEXT CHKLIST AS TO BE READY AT THE END OF THE RWY (APPROX 10 SECONDS). AS I LOOKED UP TO SEE OUR POS; I DID NOT SEE THE ALL BLUE TAXI LIGHT -- IT WAS A MIXTURE OF BLUE AND WHITE. I THOUGHT IT MIGHT BE RWY 12. AS I LOOKED TO THE R THE RWY EDGE LIGHTS DISAPPEARED FROM MY AFT R WINDOW. I ALSO SAW AN ACFT AT THE ARR END OF RWY 12 IN POS WITH ALL ITS LIGHTS ON. I STARTED TO MOVE MY HAND TOWARDS THE THROTTLES TO APPLY PWR IF IT APPEARED THE OTHER ACFT WAS MOVING. AT THIS SAME TIME I SAID TO THE CAPT; 'NO; NO; NO; WE ARE ON THE RWY WE WERE SUPPOSED TO TURN.' I DID NOT APPLY BRAKES; AS IF THE OTHER ACFT WAS MOVING I WANTED THE MOMENTUM TO MOVE QUICKLY -- IT WAS NOT. AT THIS NEAR SAME TIME; ATC ADVISED US WE HAD CROSSED AN ACTIVE RWY. THE CAPT THEN UNDERSTOOD HIS MISTAKE. WE WERE THEN INSTRUCTED TO PROCEED TO RWY 9R FOR DEP. ATC THEN ADVISED US THAT BECAUSE THE OTHER ACFT WAS POS AND HOLD THAT NO CONFLICT HAD RESULTED. THE CAPT ADVISED ME THAT WHILE HE HEARD MY AND ATC INSTRUCTIONS HE HAD IT IN HIS MIND THAT RWY 9L WAS RWY 9R. RWY 9R HAD BEEN OUR ARR RWY AND SO HE GRAVITATED TOWARD IT. HE HAD HEARD 'TAXI TO' AND SAW THE ACFT ON RWY 12; SO HE THOUGHT HE WAS CLRED TO CROSS RWY 12 AND WAS WATCHING THE HOLDING TFC. HE STATED THAT SOMETHING DID NOT SEEM RIGHT; BUT IT DID NOT BECOME NOTICED. THE CAPT'S PERFORMANCE SUFFERED BECAUSE HE DID NOT USE ALL AVAILABLE RESOURCES WHEN HE FELT UNEASY. THOSE BEING ATC; HIS DIAGRAM AND HIS FO. MY PERFORMANCE SUFFERED BECAUSE I WAS OVERCONFIDENT IN MY CAPT BASED ON HIS USUAL GREAT PERFORMANCE AND BACKGND. I FELT THAT HE KNEW THE ARPT AND DIRECTION TO PROCEED AND THAT I WOULD BE FREE TO PERFORM MY DUTIES. CORRECTIVE ACTIONS WOULD BE FOR ATC TO NOT USE THE TERM 'TAXI TO' UNLESS IT WAS USED IN SPECIFIC TO CROSS A RWY. THE CAPT SHOULD USE THE FO AS A LIVING PROGRESSIVE TAXI. ONLY TO BE RELEASED FOR OTHER DUTIES WHEN NO TURN OR XING OF RWYS IS REQUIRED. THE FO SHOULD TREAT THE CAPT WITH THE ATTITUDE THAT THE CAPT COULD ALWAYS BE UNAWARE OF INSTRUCTIONS OR LOCATION. AND ONLY GO HEADS DOWN WITH NO TURNS OR RWY XING TO BE PERFORMED.