Through his interpreter, Roberto Duran said he would expose Sugar Ray Leonard as a fraidy cat if Leonard shows a reluctance to mix it with him Friday night. "I will tell him like this, in front of all the people, he has no heart." Duran's fist was making boxing's traditional, and derisive, come-in-and-fight gesture.

But this contest as Leonard has planned it, will not be fought on Duran's terms. Big talk by the challenger will not suddenly remake the crafty Leonard into a dolt who fights the other man's fight. He did not get to be welterweight champion of the world that way, with this 27-0 record.

Leonard has, at points of his own choosing, too many weapons he can put to use to take any guff from Duran, rugged though that challenger is. He has faster hands than Duran, faster feet, faster punching moves, by far the best jab, and his own two fists may even be as fully loaded as Duran's despite the tiresome "hands of stone" legend that has attached itself to the challenger.

The betting on the fight is a bit of a paradox. In Las Vegas, Leonard is accepted as a solid 9-to-5 favorite, but in Montreal the army of boxing writers here for the contest is voicing a positive preference for Duran as winner and new champion.

This opinion is strongest among, and probably has been generated by, the writers from New York, where Duran has long been acclaimed for his ferocity and is a local idol. But New York sentiment is not always convincing. New York's writers, year after year, always favor the winner of Aqueduct's Wood Memorial to win the Kentucky Derby, even in the face of the long and wretched history of that presumption.

Speculation concerning Leonard always centers on "can he take a punch?" But it can be answered with another question: "Does he have to?" If he has never been in there before against the likes of slugger Duran, then Duran has never been in there with as shifty a chap as Leonard. Brawlers are hardly the type to find Leonard's chin; not with his speed and his obession not to be hit where it hurts.

It has been when Leonard's undefeated record was in the most peril that he has been at his fiercest and best. That is when he has resorted to long flurries with loaded fists, looking for a quick way out of his torment. It has worked for him all along the way.

Leonard is now boxing's best student. He bought a television tape recorder as soon as he could afford it, and spends hours studying the filmed reproduction of all his fights. He has probably watched more of Duran's films than Duran. He respects Duran but is not afraid of him. He has seen some things about Duran that has expanded his own confidence.

He knows Duran plays rough, in a rough business. Leonard's understanding of that could be equated with what was once said by the late jockey Sonny Workman, who was often accused of rough riding tactics. Said Workman: "There ain't no lady hookers, and there ain't no gentleman jockeys." Leonard is prepared for any behavior by Duran.

If the fight turns on physical condition during the late rounds, it could be to Leonard's advantage. The three-minute round is a comparative romp for the champion. His every sparring session has included five-minute rounds, an innnovation he thought up for himself, although not popular among less-dedicated fighters.

Leonard has no mind to let Duran crowd him into the ropes where the challenger can practice a flailing to head and body that has brought down so many of his 72 previous opponents, with only one defeat. Sugar Ray has trained to slide away from the ropes. He is no longer unschooled and he knows now how to control a fight.

Although Duran is an acknowledged knocker-out, his punches are more cumulative than the neat finishing blows Leonard hs delivered. Leonard can match all of Duran's zest for going after a wounded opponent, and maybe is more fierce. His accuracy is startling.

Duran is a busy fighter with his weaving method of getting inside where he can do damage with both hands. His task will be to get past Leonard's good jab without being cut up, or without being set up for the kind of right Leonard often likes to throw. Some even think Leonard's left is his best punch. And in close he's strong enough to pin Duran's arms and neutralize his punching power.

Leonard is a full-fledged welter-weight, in there against a former champ used to fighting lightweights he could move around. That may be remembered Friday night when Duran tries to take charge against Leonard. Duran has the "puncher's chance" but the strong preference here is a ticket on Leonard.