A DC9 FLC FLIES THROUGH THE RWY 26R LOC WHILE THE PIC IS MAKING JUDGEMENT CALLS ON THE FO'S USE OF WING FLAPS.

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OUR CLRNC WAS TO FLY HDG 230 DEGS TO INTERCEPT THE RWY 26R LOC; MAINTAIN 5000 FT UNTIL ESTABLISHED ON THE LOC; CLRED FOR THE APCH. AT THE TIME WE RECEIVED THE CLRNC; WE WERE ALSO DSNDING FROM 7000 FT TO MAINTAIN 5000 FT. AS WE WERE LEVELING; I CALLED FOR 15 DEGS OF FLAPS. AFTER SELECTING THE FLAPS; THE CAPT ASKED ME WHY I WANTED TO PUT 15 DEGS FLAPS OUT SO FAR FROM THE ARPT. I FELT PRESSURE TO ANSWER HIM AND BEFORE I KNEW IT WE GOT A TA AND WERE TOLD TO CANCEL APCH CLRNC AND TURN TO 360 DEGS FOR TFC. AS THIS WAS HAPPENING; I SAW THAT WE HAD FLOWN THROUGH THE RWY 26R LOC DURING OUR DISCUSSION. THE CAPT THEN TOOK THE AIRPLANE AND TURNED TO 360 DEGS; AFTER WHICH WE WERE GIVEN A NEW APCH CLRNC. THE CAPT GAVE THE AIRPLANE BACK TO ME AND WE LANDED WITHOUT FURTHER INCIDENT. NOTHING WAS SAID BY THE CTLR. I FEEL THAT THE CAPT USED POOR JUDGEMENT BY INITIATING CONVERSATION BELOW 10000 FT WHILE DURING AN INST APCH. I ALSO USED POOR JUDGEMENT BY GOING ALONG WITH THE CONVERSATION. SOME FACTORS WERE: #1 CAPT ON SENIORITY LIST; FIRST TRIP TOGETHER; FAMILIAR ARPT; AND CAPT NOT PAYING ATTN TO HIS FO!