LATE RWY ASSIGNMENT ON STAR TO LAX RESULTS IN COCKPIT DISTRACTION AND MISSED CROSSING ALT.

Narrative

I LEARNED A VERY GOOD LESSON REGARDING STARS SERVING MULTIPLE RWYS WITH A DIFFERENT SEGMENT FOR EACH RWY. IN THIS CASE IT WAS EITHER THE CIVET OR MITTS ARR INTO LAX -- I REALLY CAN'T RECALL. I HAD NEVER DONE EITHER ONE AND NEVER ANY SIMILAR ONE EITHER. SINCE THE TWO ARRS ARE SO SIMILAR; THE LESSON LEARNED CAN APPLY TO BOTH. I WAS PF IN THE LEFT SEAT ON A MORNING FLT. THERE WERE CLOUDS AROUND BUT WE WERE VFR. 20 OR 25 MILES ON HEC TRANSITION BEFORE CIVET (ON BOTH STARS) I ASKED THE PNF TO GET OUR RWY ASSIGNMENT FROM CTR AS I HAD NOT PROGRAMMED ONE IN THE FMS YET AND WE WERE UNDEFINED AFTER CIVET. LOTS OF RADIO CHATTER AND BEFORE WE COULD CALL WE WERE HANDED OFF TO SOCAL APCH. UPON CHKING IN WE WERE GIVEN RWY 25R BUT NOW I ONLY HAD 5 MILES TO REPROGRAM THE FMS SO IT WOULD MAKE THE PROPER TURNS. I HAD TO ENTER A RWY BUT WHEN I DID THE STAR DISAPPEARED SO I HAD TO ENTER THAT AND A TRANSITION AND THEN RESEQUENCE THE FLT PLAN SO WE WERE ON THE CORRECT LEG. I DID THIS IN BARELY ENOUGH TIME SO THAT WE MADE OUR TURN AT THE NEXT FIX (EITHER CIVET OR RUSTT) LOSING ONLY ABOUT 1/4 DEFLECTION ON THE HSI NEEDLE. JUST THEN THE PNF SAID WE WERE SUPPOSED TO CROSS THE FIX AT (15000 FT?) AND WE WERE AT 12000 FT. I WAS FEELING DUMB AND BEFORE I COULD SAY ANYTHING; THE CTLR GAVE US A DSCNT TO 10000 FT AND A HDG OF 260 DEGS TO INTERCEPT ILS RWY 25R. I WAS GLAD TO HEAR THAT! THE REST OF THE FLT WENT WELL; THOUGH I WAS EMBARRASSED. I WAS TIRED (LITTLE SLEEP) AND SHOULD HAVE HAD THE PNF DO THE FMS PROGRAMMING SO I COULD KEEP SITUATIONAL AWARENESS. I SHOULD HAVE ALSO READ THE NOTE TO SET UP FOR RWY 25L UNLESS OTHERWISE INSTRUCTED EVEN THOUGH RWY 25L WAS CLOSED; THIS WOULD HAVE SAVED US OR GIVEN ENOUGH TIME TO GET THINGS RIGHT. I COULD HAVE HAD THE PNF DETERMINE OUR RWY EARLIER; TOO. AND I COULD HAVE READ THE ARR MORE CAREFULLY ON SEGMENTS FROM CIVET TO LAX SO I'D KNOW WHAT TO EXPECT BUT I WAS GOING TO DO THAT AFTER RWY ASSIGNMENT. IN THE FUTURE; I WILL SET UP A SEGMENT OF MOST LIKELY RWY OR AS INSTRUCTED ON STAR. I WILL READ STAR MORE CAREFULLY AND WILL GET RWY ASSIGNMENT EARLIER.