COMMUTER ACFT ENTERS WRONG RWY FOR TKOF AT MIA. CLRS RWY THEN CTLR SAYS OK TKOF THAT RWY.

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WE WERE FILED FOR MTH OUT OF MIAMI. NEW FO TO THE AREA AND I THE PF WAS TAXIING. DURING THE TAXI WE WERE CLRED TO 9L. WHEN EVER WE TAXIED TO AN E RWY AND WERE FILED FOR AN ARPT TO THE S; WE WOULD GET A RWY CHANGE TO 12 99 PERCENT OF THE TIME. THIS WAS THAT 1 PERCENT THAT WE WERE CLRED FOR DEP ON 9L. I THOUGHT WE WERE CLRED TO DEPART 12 SO I TOOK RWY 12. I ALWAYS CHK FOR FINAL TFC PRIOR TO ENTERING A RWY; AND THERE WAS NONE. THE CTLR TOLD US HE ASSIGNED 9L. I IMMEDIATELY CLRED 12 AND THE CTLR SAID IT WAS ALRIGHT IF WE WANTED 12. WE WERE THEN CLRED TO DEPART 12. THERE WAS NO TFC AROUND; AND WE WERE NOT IN THE MIDDLE OF A PUSH. THIS HOPEFULLY WILL BE THE LAST TIME I EVER DO SUCH AN IGNORANT COMPLACENT THING. CONTRIBUTING FACTORS ARE COMPLACENCY; NEW FO TO THE ARPT; BREAK DOWN IN COM BTWN CREW. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. RPTR STATES NO BLAME ANYWHERE BUT ON HIS SHOULDERS. HIS COMPLACENCY IN EXPECTING THE ROUTINE OF RWY ASSIGNMENT WAS PROBLEM. FO VERY NEW AND CONCERN FOR SLOWING DOWN TO ASSURE ALL TASKS COMPLETED WITHOUT FEELING RUSHED WAS DISTRACTING. NEVER AGAIN. NEW PROC HE HAS ADOPTED TO CONFIRM RWY ASSIGNMENT PRIOR TO TAXI ONTO RWY.