Marvelous Marvin Hagler and Thomas Hearns faced each other three months earlier than their scheduled middleweight championship bout yesterday. But not a punch was thrown.

The mock ring meeting was for an Internal Revenue Service public service announcement and part of a three-week promotional campaign to sell tickets to their 12-round title fight April 15 in Las Vegas. Following yesterday's work, the fighters' entourages will visit 17 cities in 10 days and film at least three more commercials before the two begin intensive workouts.

"You don't want to get too friendly and you don't really want to, you know, love the guy, because you have to go into the ring and do a job," Hagler said. "And the only way you can do it properly so that you don't get hurt or anything is to go in there with that meanness."

Hearns and Hagler spent eight hours filming a 30-second spot at the Sugar Ray Leonard Boxing Gymnasium in Palmer Park.

"Well," Hagler said, "it's a good cause, the IRS. And it will help people file their income taxes early. The exposure is good for the both of us."

"I think it's worthwhile," Hearns said. "I understand they (the IRS) take the proceeds from the fight. Maybe they'll give me a break in the future. Nobody is perfect. I might make a mistake in the future and they'll give me a break."

Hearns and Hagler spent nearly four of their eight hours together glove to glove but, except for the script dialogue, did not speak to each other. Hearns was quiet and businesslike, while Hagler's scowl frequently turned to a smile.

"Basically, you know, there's a whole lot of stuff that's hidden under that smiling face," Hagler said.

Hearns informs the public in the commercial that, if we file early, we can all relax on April 15. Except, Hagler interjects, for Hearns.

At one point yesterday, Hearns tripped over the same line six times while reading from cue cards. When the referee subsequently missed his line, Hearns patted him with his glove and broke up the corps of photographers by telling him he was entitled to a mistake.

In the artificially smoke-filled ring, Hagler stood under the lights and complained that he needed a hat to protect his bald head from the heat. He also complained when, late in the afternoon, he had to stand in for extra takes. Otherwise, the champion appeared to thrive when Jim Brown, the director, called for action.

Brown had his 12-person film crew take 27 takes before he was satisfied.

"I was real worried at first," Brown said. "The promoters had to wonder if we were going to have problems 'cause the fighters didn't like each other and the script called for them to be funny and humorous.

"As it turned out, they were very loose and, at the end of the shoot(ing) the promoters were upset, saying we had ruined a good fight."

Promoter Bob Arum will attempt to sell 2 million tickets to watch the fight on closed-circuit TV. As part of the promotional campaign, Hagler also has posed for a March of Dimes poster.

If a minimum of $14 million can be netted from closed-circuit viewing, Hagler will receive 45 percent of the excess, on top of his fight fee of $5.3 million. Hearns will receive 35 percent on top of his $5.1 million.

So much more for the IRS.

Hagler and Hearns were set to fight once before, in May 1982, but the bout was delayed when Hearns broke a finger. Subsequent contract negotiations fell apart.

As for the promotional tour, "This is just the beginning because I think it's going to be very hectic," Hagler said. "I feel like I'd rather go on ahead into the fight than to be doing this kind of stuff, really."

When the tour is over, Hagler will return to Provincetown, Mass., to train and Hearns will prepare in Detroit. Hagler will enter the fight with a record of 60-2-2 (50 knockouts); Hearns has a record of 40-1 (34 knockouts).

"It gives heightened awareness of the fight," Arum said of the public service spots. "Of all the entities involved in promotion, the IRS has the biggest interest. They're going to make the most money of all."