A B737 TAXIING FOR TKOF AT MIA; SUFFERS A RWY INCURSION.

Narrative

DURING TAXI AT MIA; WE RECEIVED TAXI INSTRUCTIONS TO TAXI TO RWY 9L VIA TXWY Q. I HAD ALREADY PLANNED RWY 9L VIA TXWY P AND ACTUALLY BRIEFED IT BEFORE WE STARTED OUR TAXI. AS WE WERE TAXIING; I SAW THE 'Q' SIGN OFF TO OUR L; SHOWING A L TURN. AT THAT POINT; FOR SOME REASON; I THOUGHT TWR WANTED US TO TAXI TO RWY 9R -- I STOPPED THINKING RWY 9L. AS WE APCHED RWY 12/30 ON WHAT I THOUGHT WAS TXWY Q; BUT WAS ACTUALLY TXWY S; WE STOPPED THE ACFT PRIOR TO XING RWY 12/30 TO CONFIRM THAT THERE WAS NEITHER AIRBORNE TFC NOR GND TFC. THERE WAS NONE. I THEN BEGAN TAXIING. AS THE NOSE OF THE ACFT PASSED THE RED RWY SIGNS; TWR GAVE US INSTRUCTIONS TO TURN R ON RWY 12/30 AND EXIT TO TXWY Q AND TAXI TO RWY 9L; WHICH WE DID THIS TIME. WHAT WE THOUGHT WAS A XGRESSION AND WHAT MIAMI TWR; WE BELIEVE; THOUGHT WAS A XGRESSION; WAS THOUGHT TO BE SOMETHING OTHER THAN THAT BY OUR FAA INSPECTOR IN THE JUMP SEAT. WHAT ITEMS CAUSED THIS SIT WERE: 1) THE SIGN 'Q' POINTING TO THE L GOT ME THINKING RWY 9R. 2) TAXI INSTRUCTION DID NOT INCLUDE HOLD SHORT; WHICH WAS NOT NECESSARY IF I HAD ACTUALLY STAYED ON TXWY Q TO RWY 9L. 3) IT WAS MY FO'S FIRST TIME TO MIAMI.