ON STAR ARR RPTR PF; FO; WAS RESPONSIBLE FOR PA ANNOUNCEMENTS AS WELL AS FLYING SO WHEN HE WAS OFF THE COM RADIO ATC GAVE A RWY CHANGE AND ALT CHANGE. PNF CAPT DIDN'T TELL THE FO UNTIL HE HAD SET THE MCP AND DSNDED BELOW THE ASSIGNED ALT.

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I WAS THE PF; IN ADDITION CAPT HAD REQUESTED AT BEGINNING OF TRIP THAT I DO ALL OF THE PA'S. KNOWING THIS I ATTEMPTED TO COMPLETE PA EARLY; HOWEVER; FLT ATTENDANTS WERE DOING THEIR ARR BRIEFING. I WAS NOT ABLE TO DO MY BRIEFING UNTIL LEAVING APPROX 14000 FT. THE LAST CLRNC I HEARD WAS CLRED CIVET 1 ARR CROSS ARNES AT 250 KTS. I COMPLETED PA AS THE ACFT REACHED BREMR AT 12000 FT. I ANNOUNCED THAT I WAS BACK ON THE COM RADIO. I ENTERED 10000 FT IN THE MCP ALT WINDOW AND STARTED DOWN IN VERT SPD TO 10000 FT AT ARNES. AS THE ACFT DSNDED THROUGH 11600 FT THE CAPT SAID WE WERE TO MAINTAIN 12000 FT. I RESPONDED BY DISENGAGING THE AUTOPLT AND CLBING BACK TO 12000 FT. I SAID THIS IS THE FIRST TIME I HEARD THAT CLRNC. THE CAPT RESPONDED BY SAYING; HE GAVE THAT TO US WHILE YOU WERE DOING THE PA (THE ACFT REACHED 11500 FT). ONCE LEVEL AT 12000 FT THE CAPT ASKED APCH IF THEY STILL NEEDED US AT 12000 FT. THEY SAID NO; DSND TO 8000 FT. ALSO I ALMOST FORGOT IN THE INITIAL DSCNT TO 10000 FT WE WERE GIVEN A RWY CHANGE FROM RWY 25L TO RWY 24R. I'M SURE THIS PLAYED A PART IN THE CAPT FORGETTING TO TELL ME THE CLRNC HAD CHANGED. APCH NEVER SAID ANYTHING REGARDING OUR ALTDEV. THE REMAINDER OF THE FLT WAS NORMAL.