A320 CREW LEAVES ALT ON STAR BEFORE THEY ARE CLRED.

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WE HAD BEEN CLRED THE PDZ 3 ARR RWY 25L INTO LAX AND TOLD TO CROSS KONZL AT 17000 FT. WE LEVELED AT 17000 FT PRIOR TO KONZL AND WERE BOTH EXPECTING OUR NEXT CLRNC TO BE 'DSND ON THE PDZ 3 ARR.' AT JUST PAST KONZL; THE CTLR TOLD US 'YOU ARE NOW CLRED THE PDZ 3 ARR TO RWY 24R.' I WAS THE PNF AND ACKNOWLEDGED THE CLRNC. I WENT 'HEADS DOWN' AND ACTIVATED THE SECONDARY FLT PLAN. WHEN I COMPLETED THAT TASK; I LOOKED UP AND IMMEDIATELY NOTICED THE ALT SELECT WINDOW INDICATED '8000 FT.' I GLANCED AT THE ALTIMETER AND WE WERE IN A DSCNT PASSING 16700 FT. I INFORMED HIM WE WERE NOT CLRED TO DSND AND TO IMMEDIATELY CLB BACK TO 17000 FT. OUR LOWEST ALT WAS 16500 FT. LEVEL AT 17000 FT; I ASKED APCH FOR A DSCNT CLRNC AND RECEIVED A CLRNC. TO 14000 FT WITH NO COMMENT ABOUT OUR ALTDEV. WE WERE IN AN 'ANTICIPATION' MODE GETTING HIGH ON THE PROFILE AND EXPECTING A CLRNC. WHEN THE FO HEARD THE CLRNC HE REACTED ON WHAT HE EXPECTED. 2 COMPANY PROC VIOLATIONS; NOT CONFIRMING WITH PNF NEW ALT AND SETTING THE LOWEST STEPDOWN FIX NOT THE NEXT. MY 'SEAT-OF-THE-PANTS' DID NOT DETECT THE DSCNT WHEN I WAS HEADS DOWN.