CREW ON B737 AT CLT PARKED AT THE GATE WERE PLANNING TO USE RWY 18. ON PUSHBACK AND CONTACT WITH GND CTL; THEY WERE GIVEN RWY 36 AND TO FOLLOW ACFT AHEAD TO SPOT X. ACFT AHEAD CROSSED RWY 5 AND THEY FOLLOWED WITHOUT CLRNC TO CROSS.

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AFTER PUSHBACK FROM GATE WE CONTACTED CLT RAMP CTL. THEY SAID FOLLOW ACFT AHEAD TO SPOT X AND CONTACT GND POINT Y FOR RWY 36L. THIS CREATED SOME CONFUSION AND ADDITIONAL WORK BECAUSE WE HAD PREVIOUSLY BRIEFED USING RWY 18 WHILE WE WERE PARKED AT THE GATE. THE CAPT SAID; 'ARE WE GOING TO RWY 36L?' I SAID; 'YEAH; I GUESS THEY'RE USING RWY 36 NOW.' WE STARTED DISCUSSING WHAT THE HDG AFTER TKOF WOULD BE USING THE NEW RWY. I STARTED LOOKING AT CHARTS TO CONFIRM THIS NEW HDG. WHILE TAXIING WE FOLLOWED THE ACFT AHEAD WHICH WAS NOW XING RWY 5 INSTEAD OF STOPPING AT SPOT X AND CONTACTING GND AS WE WERE INSTRUCTED TO DO. WE HAD INADVERTENTLY CROSSED RWY 5 WITH NO CLRNC TO DO SO. PROB WAS DISCOVERED WHEN GND CONTACTED US AND GAVE US A PHONE NUMBER TO CALL THEM AFTER LNDG. UPON ARR IN MCO THE CAPT CONTACTED THE CTLR IN CLT TO DISCUSS THE SIT. NO CONFLICT AROSE BECAUSE OF OUR UNAUTH XING OF RWY 5. I FEEL CORRECTIVE ACTION WOULD BE TO NOT REBRIEF TKOF UNTIL ACFT IS STOPPED. NEVER DO ANYTHING TO DIVERT YOUR ATTN WHILE TAXIING UNTIL ACFT IS CLR OF ALL RWYS; AND IS STOPPED WITH THE PARKING BRAKE SET.