B757; CIVET STAR LAX; FO; PF; FAILED TO SELECT RWY IN USE. CAPT SELECTED RWY 25L AFTER FO HAD SET UP LNAV DIRECT DOWNE. WHEN CAPT SELECTED RWY 25L FMC; THE ACFT STARTED 180 DEGS TO RETURN TO ARNES. CAPT REALIZED FO WAS BEHIND ACFT AND MANAGED TO REPROGRAM AND COMPLETE CHKLIST FOR A SUCCESSFUL LNDG.

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3 MINS PRIOR TO TOP OF DSCNT; CAPT HAD TO LEAVE FLT DECK TO ATTEND TO PHYSIOLOGIC NEEDS. FO WAS PF FOR LEG AND HAD CIVET ARR PROGRAMMED IN FMS PRIOR TO CAPT LEAVING. HE HAD NOT SELECTED THE RWY AT THAT TIME. WHEN CAPT RETURNED; ACFT WAS PASSING FL280 TO MAKE 140A180B AT CIVET; LNAV/VNAV WAS IN USE. WE WERE CTRED ON LOC COURSE ON 109.9; BUT FO HAD NOT SELECTED EITHER HSI TO 'ILS' MODE SO AS TO RECEIVE DME AND NEITHER VOR WAS IN MANUAL -- TOTAL RELIANCE ON THE 'MAGIC.' CAPT BRIEFLY SELECTED ILS AND VERIFIED DME/LOC CTRED AND WAS OCCUPIED IN COMPLETING APCH SETUP IN FMS; GETTING ATIS; PLANNING DSCNT CHK AND BRIEFING THE APCH FOR FO. FO WAS BEHIND AND NOT CATCHING UP. SEQUENCE IN FMS PROGRAMMING WAS SELECTING RWY 25L; LNAV DIRECT DOWNE; THEN BACK TO DEP/ARR PAGE TO RESELECT RWY 25L; CIVET; ARNES TRANSITION. THIS DROPPED DOWNE AS ACTIVE WAYPOINT; AND STARTED JET IN TURN BACK TO ARNES. FO DID NOT HAVE LOC SELECTED!? LAX APCH GAVE A TURN TO 220 DEG TO REJOIN RWY 25L LOC. THAT'S WHEN CAPT NOTED FO'S STATE OF 'BEHINDNESS;' AND TALKED HIM THROUGH THE STEPS NECESSARY TO COMPLETE APCH 'RAW DATA.' AFTER THAT; THERE WAS BARELY ENOUGH TIME TO COMPLETE THE CHKLISTS; BRIEF THE APCH; AND GET THE JET ON THE GND. FO HAS ERRONEOUSLY SELECTED APCH PRIOR TO 10 DME; PER NOTAMS WHEN INSTRUCTION HAD BEEN SELECT LOC AND GOT LOW (AS A CONSEQUENCE OF THAT) BY 200 FT AT HUNDA (1 DOT LOW). CAPT WAS TASK SATURATED AS WELL; TRYING TO SUPPORT FO AND NOT TAKE THE ACFT OR TAKE US OUT OF SEQUENCE SINCE MANY PAX HAD TIGHT INTL CONNECTIONS (ALREADY 30 MINS LATE). THAT WAS THE CAPT'S MISTAKE; ALTHOUGH WE PULLED IT OUT THIS TIME. BUT BOY; IT WAS NOT PRETTY!! CAPT AND FO WERE BOTH UNDER 100 HRS AND NEW TO JET -- PLAYED SOME PART IN EVENTS DESCRIBED.