ACR CREW ENTERS RWY AT HOU; TX; WITHOUT CLRNC CAUSING ANOTHER ACFT TO GO AROUND.

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WE WERE 8 MINS LATE PUSHING BACK FROM THE GATE. WHEN I CALLED FOR TAXI; GND CLRED US TO TAXI TO RWY 12R VIA TXWY E AND HOLD SHORT OF RWY 12L. WHILE WE WERE DOING OUR PRE TKOF CHKLIST; THE CAPT CALLED ON GND FREQ TO SWITCH TO TWR FREQ. AFTER BEING QUERIED TWICE; THE GND CTLR SENT US OVER TO TWR FREQ. THE TWR CTLR WAS VERY BUSY; BUT WITH NO HESITATION ISSUED US CLRNC TO TAXI TO RWY 12R. I READ BACK THE CLRNC AND THE CAPT ASKED ME FOR THE 'BELOW THE LINE CHKLIST.' THIS WAS A BIT UNUSUAL SINCE WE NORMALLY WAIT UNTIL WE ARE CLRED INTO POS AND HOLD OR CLRED FOR TKOF; BEFORE WE ACCOMPLISH THIS FINAL PART OF THE CHKLIST. BUT SINCE WE HAD SUCH A SHORT TAXI TO THE ACTIVE RWY; I ASSUMED THAT THE CAPT WANTED TO GIVE OUR FLT ATTENDANTS AS MUCH TIME AS POSSIBLE TO GET THE CABIN READY FOR TKOF. I COMPLETED MY FLOW AND BEGAN TO READ THE 'BELOW THE LINE CHKLIST' TO THE CAPT AND AS I WAS READING IT TO HIM; HE CONTINUED TO TAXI. AT THIS POINT; WE WERE APCHING THE HOLD SHORT LINE AT RWY 12R AT A BRISK TAXI SPD. I NOTICED THAT WE DID NOT START DECELERATING TO HOLD SHORT OF RWY 12R. I WAS TRYING TO SORT OUT; IN MY HEAD; WHETHER I HAD MISSED A RADIO CALL OR WHETHER THE CAPT WAS NOT COMPLYING WITH OUR CLRNC. MY NORMAL HABIT PATTERN WHEN WE ENTER AN ACTIVE RWY IS TO CLR THE APCH SECTOR FOR OTHER ACFT. WHEN I DID THIS; I SAW AN ACFT ON APPROX 2 1/2 MI FINAL. I TOLD MY CAPT ABOUT THE ACFT AND THE FACT THAT I WAS UNSURE OF OUR CLRNC ONTO THE RWY. BY THIS TIME WE WERE ON THE ACTIVE RWY WITH MOST PART OF THE ACFT AND WERE BEGINNING TO DECELERATE. WITHOUT HESITATION THE CAPT RADIOED TWR AND WAS TOLD THAT WE HAD NOT BEEN CLRED ONTO THE RWY; TO EXIT RWY 12R AND TAXI ONTO RWY 17; TURN L ON RWY 17; AND TO HOLD SHORT OF RWY 12R. THE CTLR SENT THE ACFT ON FINAL AROUND AND WE IMMEDIATELY EXITED RWY 12R ACCORDING TO THE CTLR'S INSTRUCTIONS. AFTER 2 ADDITIONAL ARRS WE WERE CLRED FOR TKOF AND PROCEEDED UNEVENTFULLY TO DALLAS LOVE. I BELIEVE THERE ARE 5 CONTRIBUTING FACTORS TO THIS OCCURRENCE: 1) WE LEFT THE GATE 8 MINS LATE; WHICH CAUSED AN UNNECESSARY URGENCY TO MAKE UP FOR LOST TIME. 2) THE VERY SHORT TAXI DISTANCE FROM OUR GATE TO THE ACTIVE RWY COMBINED WITH THE ABOVE-MENTIONED URGENCY LEFT NO ROOM FOR ERROR OR DOUBLECHKING THE OTHER CREW MEMBER'S ACTIONS. 3) AN UNEXPECTED RADIO CALL. WHEN WE SWITCHED TO TWR FREQ; WE WERE EXPECTING WHAT WE NORMALLY HEAR; WHICH IS 'CROSS RWY 12L ON TXWY E AND HOLD SHORT OF RWY 12R.' THE TWR ACTUALLY SAID 'TAXI TO RWY 12R;' WHICH DOES IMPLY TO HOLD SHORT OF THE RWY. I BELIEVE THAT BECAUSE THE CAPT DID NOT HEAR WHAT HE EXPECTED 'CROSS RWY 12L ON TXWY E AND HOLD SHORT OF RWY 12R;' BUT INSTEAD HEARD 'TAXI TO RWY 12R' THAT HE PERCEIVED THAT AS CLRNC ONTO RWY 12R; WHICH WAS NOT CORRECT. 4) TWR FREQ WAS VERY CONGESTED AND THE CTLR WAS ISSUING CLRNCS AT A VERY FAST PACE. 5) A LOT OF THINGS WERE HAPPENING IN A VERY SMALL AMOUNT OF TIME; WHICH RESULTED IN A HIGH WORKLOAD FOR THE FLC. THESE ARE JUST SOME OF THE LESSONS THAT I HAVE LEARNED FROM THIS EVENT: 1) SLOW DOWN AND GIVE THE OTHER CREW MEMBER A CHANCE TO CATCH UP SO THAT THERE IS TIME FOR HIM TO VOICE HIS CONCERNS BEFORE IT IS TOO LATE TO STOP. 2) DO NOT GET COMPLACENT JUST BECAUSE YOU HAVE BEEN AT AN ARPT MANY TIMES AND THINK YOU KNOW WHAT CLRNC TO EXPECT NEXT. 3) BE EXTRA VIGILANT DURING TIMES OF FREQ CONGESTION; SHORT TAXI RTES AND AROUND RWYS THAT ARE IN CLOSE PROX TO EACH OTHER (HOUSTON HOBBY RWY 12R; RWY 12L; RWY 17). 4) RELY ON YOUR ESTABLISHED HABIT PATTERNS. XCHKING THE FINAL APCH SECTOR PROBABLY HELPED TO PREVENT THIS SIT FROM DEVELOPING INTO A POSSIBLE ACCIDENT. 5) DO NOT HESITATE TO SPEAK UP IMMEDIATELY; IF YOU ARE UNSURE OF A CLRNC OR UNSURE OF THE OTHER CREW MEMBER'S UNDERSTANDING OF A CLRNC.