HDG TRACK ALTDEV DURING IAP ILS APCH. ALT OVERSHOT IN DSCNT. POTENTIAL CONFLICT LTSS.

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FO WAS FLYING THE ACFT THIS LEG. HE HAS BEEN RETURNED TO FLYING VERY RECENTLY AFTER HAVING BEEN ON MEDICAL LEAVE FOR OVER 6 YRS. I DON'T THINK HE HAS MORE THAN 50 HRS ON LINE OP; BUT THAT IS AN ESTIMATE. ATL APCH ISSUED A TURN TO 270 DEGS (FROM OUR PREVIOUSLY ESTABLISHED 200 DEG HDG) AND TO MAINTAIN 5000 FT AND CLRED FOR THE ILS (OR VISUAL) APCH. I HAD SEEN TFC AT OUR 12 O'CLOCK POS AND WAS KEEPING MY EYE ON HIM SINCE WE WERE CLOSING QUICKLY. INITIALLY I THOUGHT WE WERE FOLLOWING HIM TO 26R. HE WAS GOING TO 27R OR L. IT DID NOT SEEM PECULIAR; NOR WAS I SUSPICIOUS THAT SOMETHING WAS AMISS UNTIL I HEARD THE ALT ALERTER WARNING. (ATC HAD TOLD US TO MAINTAIN 5000 FT UNTIL ESTABLISHED.) THEN I REALIZED MY FO HAD FAILED TO TURN FROM 200 DEGS TO 270 DEGS; FAILED TO INTERCEPT THE LOC; AND FAILED TO LEVEL OFF AT 5000 FT! THE TFC I WAS WATCHING WAS ESTABLISHED ON A LOC TO A DIFFERENT RWY. ON OUR 200 DEG HDG; WE WERE CLOSING AND I WAS CONCERNED ABOUT ESTABLISHING SPACING IN SEQUENCE. THIS ENTIRE INCIDENT PROBABLY DIDN'T TAKE 20 SECONDS. HAD MY FO JUST LEVELED AT 5000 FT AND ESTABLISHED A TURN; I WOULD HAVE NOTICED THAT WE WERE NOT TO FOLLOW THE TFC -- WE WERE SUPPOSED TO PARALLEL THEM. I HAVEN'T GOT ANY IDEA WHAT THE FO WAS DOING. HE HAS BEEN OUT OF THE SYS SO LONG THAT THESE SITUATIONS OVERLOADED HIM. I SHOULD HAVE KNOWN BETTER; BUT I CAN'T ALWAYS DO IT ALL -- LEVEL OFF; TURN AND INTERCEPT; ALL SIMULTANEOUSLY; WITH TFC 12 O'CLOCK AND 2 MI (OR 1 1/2 MI) IS A LOT TO DO. THAT'S WHY THERE ARE 2 OF US. SUDDENLY; HOWEVER; I FELT ALL ALONE.