MD83 INBOUND TO LAX DSNDS BELOW ASSIGNED ALT AND VARIES OFF COURSE WHILE TRYING TO PROGRAM COCKPIT FOR NEW RWY ASSIGNMENT.

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UPON CLRNC TO FL180 FROM FL240; THE CAPT (WHO WAS THE PF) TURNED THE AUTOPLT OFF AND HAND FLEW THE AIRPLANE. WE LEVELED AT FL180 WITH ALT HOLD ON THE FMA. WE WERE LESS THAN 20 MI FROM THE CIVET INTXN AND WERE CLRED VIA THE MITTS ARR. THIS ARR GOES AS FAR AS CIVET AND THEN BECOMES RWY SPECIFIC. I ASKED ATC WHICH RWY WE WERE TO EXPECT. HE SAID TO FLY THE MITTS ARR AND CONTACT APCH CTL. ABSENT A RWY ASSIGNMENT; WE HAD RWY 25L IN THE FMS. AFTER CONTACT WITH APCH CTL; I ASKED WHICH RWY TO EXPECT. AFTER A SHORT DELAY; WE WERE GIVEN THE MITTS ARR RWY 24R. WE WERE CLOSE TO CIVET AND THE FMS WAS THEN STARTING TO TRACK THE RWY 25L TRANSITION. I LOOKED DOWN AT THE ARR CHART TO FIND THE INTERCEPT FROM SMO AND SET IT IN AS RAW DATA BACK-UP. THEN I NOTICED WE WERE 300 FT BELOW FL180. I SAID 'ALT' AND THE CAPT CLBED BACK TO FL180. HE HAD LOOKED DOWN TO CHANGE THE FMS OVER TO RWY 24R WHILE HAND FLYING THE AIRPLANE. THEN THE CTLR GAVE US DSCNT CLRNC ON THE MITTS ARR. THE CAPT WAS STILL TRYING TO PROGRAM THE FMS. I THEN BEGAN PROGRAMMING THE FMS AND PUTTING IN THE WAYPOINTS. HIS ATTN WAS STILL ON THE FMS. WE WERE NOW N OF THE RWY 24R LOC. I COULD SEE THE ARPT AND TOLD HIM THE LOC WAS TO THE L. HE ACKNOWLEDGED BUT DID NOT CORRECT TOWARDS IT. WE WERE NOW HIGH ON THE ARR. THE CTLR FINALLY GAVE US A 30 DEG HDG TO THE L AND TOLD US TO INTERCEPT THE RWY 24R LOC; MAINTAIN 8000 FT AND CLRED US FOR THE APCH. WE WERE LATER CLRED FOR A VISUAL APCH BEHIND TFC. THE CTLRS NEVER SAID ANYTHING ABOUT ANY TFC OR ALT CONFLICTS DURING THIS ARR. I DID NOT HEAR THE CTLR ISSUE ANY INSTRUCTIONS TO OTHER ACFT IN REF TO OUR DEVS. THEY DID NOT ASK US TO CALL THEM. THE #1 LESSON REINFORCED HERE IS TO FLY THE AIRPLANE. ALWAYS. #2 IS TO USE THE AUTOPLT DURING HIGH WORKLOAD SITS. IN THE PAST; I HAVE HEARD OTHER ACFT ASK ATC ABOUT THIS ARR. ONE CTLR'S RESPONSE WAS; 'ALWAYS TO RWY 25L.' I DON'T THINK THAT IS ALWAYS THE CASE. IF THE CTR CTLR WAS ABLE TO TELL US WHICH RWY TO EXPECT; WE WOULD HAVE HAD TIME TO PROPERLY SET THE FMS. WITHOUT THAT TIME; I SHOULD HAVE SUGGESTED THE FMS BE TOTALLY DISREGARDED UNTIL COURSE WAS ESTABLISHED USING VOR/LOC GUIDANCE. LASTLY; I COULD HAVE SUGGESTED WE REFUSE THE ARR AT THAT POINT AND ASKED FOR A VECTOR TO ESTABLISH OUR COURSE INBOUND AND THEN PICK UP THE ARR.