WHILE STRUGGLING TO BOTH FLY A VISUAL APCH TO A BARELY VISIBLE RWY AND MAINTAIN SEPARATION WITH PRECEDING ACFT FLT CREW OF B737 DEPARTED FROM EXTENDED RWY CTRLINE. PNF ALERTED PF AND THEY RETURNED TO AN ON COURSE CONDITION.

Narrative

ON R DOWNWIND TO BASE FOR LAX RWY 24R; WITH CEILING AND VISIBILITY RESTR; APCH CALLED A SAAB 340 OFF OUR NOSE AND ASKED IF WE HAD THE FIELD IN SIGHT. THE CAPT (PF) ACKNOWLEDGED THAT HE HAD BOTH THE ARPT AND THE SAAB AND I RPTED THIS TO APCH. THEY TOLD US TO FOLLOW THE SAAB 340 FOR A VISUAL APCH TO RWY 24R AND TO CONTACT TWR. THE CAPT; WHILE SLOWING RAPIDLY; ROLLED INTO A R HAND TURN FOR THE BASE TO FINAL INTERCEPT BEHIND THE SAAB AND CALLED FOR FLAPS 5 DEGS. I LOOKED DOWN TO SELECT FLAPS 5 DEGS AND SWITCH RADIO FREQS TO TWR. WHEN I LOOKED BACK UP; THE PF HAD DECREASED HIS BACK ANGLE TO ALMOST WINGS LEVEL. I IMMEDIATELY LOOKED OVER AT HIM AND ASKED HIM IF HE STILL HAD THE RWY IN SIGHT; AT WHICH HE TIGHTENED HIS BANK TO REALIGN WITH THE LNDG RWY. I KNEW FROM APCH THAT THEY HAD INBOUND TFC FOR THE S RWYS AND I HAD A VISUAL ON AN AIRBUS S OF OUR POS WHICH COULD HAVE BEEN A FACTOR IF WE HADN'T TURNED BACK R WHEN WE DID. APPARENTLY THE PF HAD TAKEN HIS EYES OFF THE RWY WHILE HE WAS ESTABLISHING HIS SPACING ON THE SAAB AHEAD AND DIDN'T REALIZE THAT HE HAD GONE THROUGH CTRLINE. NEITHER TWR NOR GND SAID ANYTHING EXCEPT TO ASK US IF WE WANTED RWY 24L WHEN WE WERE TOO CLOSE IN TO SAFELY SIDESTEP. THE SAAB HAD APPARENTLY SLOWED TO HIS MINIMUM FINAL APCH SPD WELL OUTSIDE THE FAF AND THEN ROLLED ALMOST TO THE END OF RWY 24R BEFORE TURNING OFF THE RWY. LESSONS LEARNED: DON'T LOOK DOWN AT A CRITICAL TIME; EVEN WITH A SENIOR AND VERY EXPERIENCED PF. DON'T ASSUME THAT THE PF IS LOOKING AT THE SAME THING AS THE PNF. ABOVE ALL; EXPECT OTHER ACFT TO DO THE UNEXPECTED.