AN EMB145 FLC LINES UP WITH RWY 09 AFTER APCH CTLR HAD RECLEARED THE FLT FOR AN APCH TO RWY 08 AT IAH; TX.

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THE FLT DESCENDED ON THE ROCKIT ARRIVAL. WHEN WE WERE HANDED OFF TO APCH CTL. WE WERE TOLD TO EXPECT ILS RWY 9. THIS REQUIRED US TO REBRIEF THE APCH AS WE HAD ALREADY BRIEFED RWY 8. WE WERE ISSUED A FREQ CHANGE AS WE FLEW ABEAM THE ARPT. WE WERE ISSUED A TURN OF 170 DEGS AND SUBSEQUENTLY ISSUED A CLRNC FOR ILS RWY 8; WHICH WE APPARENTLY READ BACK WITHOUT QUERY AS WE WERE EXPECTING RWY 9. WE STILL JOINED RWY 9; NOT CATCHING IT AS WE WERE NEVER TOLD OUR APCH WAS CHANGED. IN THE FOLLOWING MOMENTS WE DID DISCUSS WHETHER WE HEARD RWY 8 OR RWY 9 AND WHAT WE READ BACK. WE SHOULD HAVE ASKED ATC AT THE EARLIEST MOMENTS OF THE APCH AND THE WHOLE EVENT COULD HAVE BEEN AVOIDED. TOWER ASKED WHAT APCH WE WERE ON AND QUICKLY CLRED US TO LAND ON RWY 9; WE THEN CONTINUED FOR 10-20 SECS. WE WERE THEN TOLD TO TURN LEFT AND GIVEN A CLRNC TO LAND ON RWY 8; AND LANDED NORMALLY. WE LEARNED LATER; SPEAKING TO A TRACON INDIVIDUAL THAT ANOTHER AIRPLANE BEHIND US WAS MADE TO GAR; AND PROPER SEPARATION WAS MAINTAINED. SUPPLEMENTAL INFO FROM ACN 535569: ALTHOUGH ATC'S INITIAL INSTRUCTION TO EXPECT THE SOUTH RWY WAS A CONTRIBUTING FACTOR; THIS EVENT COULD HAVE BEEN AVOIDED IF WE HAD LISTENED UP AND QUERIED ATC; BEFORE THEY QUERIED US; ON CONTACT WITH IAH TOWER AS THAT POINT THERE WERE SIGNALS THAT SOMETHING WAS WRONG. I PLAN TO DISCUSS THIS EVENT I DETAIL WITH MY FIRST OFFICER AND RPT IT TO OUR AIRLINE'S SAFETY COMMITTEE. I HAVE FLOWN THE APCHS TO THIS ARPT HUNDREDS OF TIMES AND ALTHOUGH FLTS ARRIVING FROM THE NORTH ARE USUALLY ASSIGNED THE NORTH RWY UNDER PARALLEL APCH CONDITIONS; IT IS NOT UNUSUAL TO BE SENT TO THE OTHER SIDE IF THERE IS A LOT OF TFC. THIS WAS A 'HEADS UP' I SHOULD HAVE NOTICED MANY MILES AWAY. THIS EVENT SERVES AS A STRONG REMINDER THAT WHAT YOU EXPECT CAN BECOME AN 'INCORRECT THEORY OF THE SIT;' TO USE CRM TERMS; ESPECIALLY UNDER PERFECT ROUTINE CONDITIONS. IT IS OBVIOUS THROUGH THAT ATC STD PRACTICE OF ADVISING ACFT IF THE APCH TO EXPECT HAS CHANGED IS EXTREMELY IMPORTANT. NOT DOING SO PUT US IN A POS WHERE UNDER HIGHEST WORKLOAD MISTAKING ONE RADIO CALL COULD HAVE CAUSED A MAJOR PROB.