B727 ISSUED GAR WHEN ACFT PRECEDING DOESN'T CLR RWY. PLT QUESTIONS SPACING ON APCH AND MISSED APCH PROC WHEN CLRED TO HOLD SHORT OF XING RWY.

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THE CAPT WAS FLYING THE APCH. THE APCH WAS BRIEFED PRIOR TO DSCNT. MIA WX WAS VFR; THERE WERE TSTMS IN THE AREA. WE WERE ON VECTORS TO THE ILS RWY 9R. MIA APCH HAD SLOWED US TO FOLLOW AN ATR. ACCORDING TO TCASII; WE WERE A LITTLE MORE THAN 2 MI BEHIND THE ATR. WE DID NOT RPT THE ATR IN SIGHT TO APCH OR THE TWR. AFTER BEING TURNED OVER TO TWR; I SLOWED TO APCH SPD. THE TWR CLRED THE ATR TO LAND AND REQUESTED THAT THEY EXPEDITE THE APCH AND CLRING THE RWY. WE WERE CLRED TO LAND RWY 9R HOLD SHORT OF RWY 12. THE ATR MISSED THE TURNOFF AND TWR TOLD US TO GO AROUND. DURING OUR GAR; THE FO ASKED THE TWR FOR GAR INSTRUCTIONS. THE TWR GAVE US RWY HDG; MAINTAIN 3000 FT; AND WE COMPLIED. THE GAR; VECTORS FOR A SECOND APCH AND LNDG ON RWY 9R OCCURRED WITHOUT INCIDENT. IN REVIEWING THE PREVIOUS EVENTS RATHER THAN REQUESTING GAR INSTRUCTIONS FROM THE TWR I THINK I SHOULD HAVE FLOWN THE SPECIAL GAR PROC FOR RWY 9L HOLD SHORT OF RWY 12; THUS AVOIDING THE POSSIBILITY OF ANY UNNECESSARY CONFUSION WITH THE TWR. A MORE THOROUGH APCH BRIEFING BY ME WOULD HAVE FACILITATED THIS.