A B757 CREW OVERSHOT DEN RWY 16L LOC BECAUSE THE FLT DIRECTOR WAS NOT ARMED AND RECEIVED A TCAS RA FROM THE ACFT ON DEN RWY 16R APCH.

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CAPT PF; DURING THE DSCNT AND APCH TO FINAL INTO DENVER INTL ARPT. WE WERE ARRIVING FROM THE E ON RADAR VECTORS TO THE ARPT. WX WAS CAVU WHILE ON A MODIFIED L BASE TO FINAL TO RWY 16L. I CALLED 'FIELD IN SIGHT' AND ATC ASSIGNED US A HDG TO INTERCEPT FINAL TO RWY 16L DUE TO ARRIVING TFC FROM THE W WHICH WILL BE LNDG ON THE PARALLEL RWY 16R. WE WERE THEN CLRED FOR THE VISUAL APCH TO RWY 16L WITHOUT ANY FURTHER MENTION OR ADVISORIES ABOUT THE OPPOSITE DIRECTION TFC (CRJ) WHICH WOULD BE LNDG ON THE PARALLEL RWY. I ASSUMED THAT SINCE BOTH THE CAPT AND I HAD VISUAL CONTACT WITH THE RWY THAT HE WAS GOING TO VISUALLY FLY THE ACFT TO INTERCEPT FINAL TO RWY 16L. UNBEKNOWNST TO ME; HE WAS GOING TO WAIT TO FLY THE FLT DIRECTOR WHEN IT CAPTURED THE LOC. HOWEVER; HE HAD FORGOTTEN TO ARM THE LOC AND GS (SELECT APCH). I THEN NOTICED THAT WE WERE NOW FLYING THROUGH THE LOC/RWY CTRLINE FOR RWY 16L AND BEGINNING TO ENCROACH IN THE RWY 16R AIRSPACE. I THEN IMMEDIATELY STATED TO HIM THAT WE WERE FLYING THROUGH OUR RWY CTRLINE AND HE THEN NOTICED HE HADN'T ARMED UP THE APCH. HE THEN AGGRESSIVELY TURNED THE ACFT BACK TOWARD OUR RWY. IT WAS AT ABOUT THIS TIME THAT WE BEGAN TO RECEIVE A TCAS RA 'MONITOR VERT SPD' (ALTHOUGH I DON'T RECALL THE EXACT RA VERBIAGE). HOWEVER; OUR VSI'S DISPLAYED ALL RED FOR CLB. WE NEVER MADE ANY VISUAL CONTACT WITH THE OTHER ACFT AND WE WERE NOT CERTAIN THAT THE OTHER ACFT HAD US VISUALLY EITHER. BOTH ACFT LANDED WITHOUT FURTHER INCIDENT AND NEITHER ATC NOR THE OTHER ACFT INVOLVED MENTIONED A SINGLE WORD ABOUT THE CONFLICT.