POTENTIAL CONFLICT LTSS. THE FLC HAD THE TRANSITIONING ACFT IN SIGHT; BUT THE CAPT LOST SITUATIONAL AWARENESS AND ALMOST HIT IT.

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ON THE VISUAL APCH TO TEB; TWR ADVISED US OF OUR TFC TO FOLLOW AND ALSO A SINGLE ENG CESSNA TRANSITIONING THE ATA AT 1300 FT IN OUR 2 O'CLOCK POS. I IDENTED THE TFC AND POINTED IT OUT TO THE CAPT WHO WAS FLYING. HE CONFIRMED THE TFC IN SIGHT. SO I STARTED LOOKING FOR OUR TFC TO FOLLOW. THE SINGLE ENG CESSNA WAS APPROX 2 MI AHEAD; R TO L; ACROSS OUR DOWNWIND LEG. AS WE CONVERGED CLOSER; TWR AGAIN ADVISED US OF THE TFC (THE CESSNA) AND I RECONFIRMED WITH THE CAPT THAT HE HAD IT IN SIGHT. AT THIS TIME WE WERE CLRED TO LAND AND I WAS DISTRACTED BY THE LNDG CHKLIST. TWR AGAIN CALLED THE CESSNA TFC TO US AS WE WERE TURNING BASE. I LOOKED UP AND SPOTTED HIM AT 12:30 O'CLOCK POS AND APPROX THE SAME ALT. SEEING A POTENTIAL CONFLICT I TOLD THE CAPT 'XXX YOU'RE TOO CLOSE!' WHILE POINTING AT THE TFC. AT THIS TIME THE CAPT STEEPENED THE TURN AND INCREASED THE DSCNT. I DOUBT THAT WE WOULD HAVE COLLIDED WITHOUT THE EVASIVE ACTION; BUT IT WOULD HAVE BEEN CLOSE. SEVERAL THINGS CONTRIBUTED TO THIS INCIDENT: 1) THE USUAL CROWDED NEW YORK AIRSPACE. 2) A SLOW MOVING ACFT TRANSITIONING THE TFC AREA AT PATTERN ALT. 3) THE DIFFICULTY IN SPOTTING ACFT AGAINST THE CITY LIGHTS. LOOKING FOR THE TFC TO FOLLOW. DIFFERENTIATING BTWN TARGETS. (THERE WERE AT LEAST 2 OTHER TARGETS IN VISUAL AREA OF THE SINGLE ENG CESSNA.) 4) HABIT PATTERNS ON WHEN AND WHERE TO FLY THE TFC PATTERN (IE; TURNING BASE AT THE SAME EXACT SPOT). 5) MILD COMPLACENCY. 6) HIGH COCKPIT WORKLOADS IN THE PATTERN. 7) CHANGE IN SLEEP PATTERN (IT WAS THE FIRST NIGHT BACK TO WORK AFTER THE WKEND). THE BEST THING THAT CAN BE DONE TO PREVENT SITS LIKE THIS IS TO STAY HEADS UP AND FLY DEFENSIVELY.