Robert McCord's whole body bounces as he moves his baton to lead the Fairfax Choral Society through Handel's Messiah.

"And his name shall be called . . . Wonderful!" about 100 singers shout.

"You're really doing kind of well," McCord says, to a chorus of moans. "Let's try again."

This is the eighth time members of the amateur group have prepared for a Messiah concert in the society's 20 years; they are long past treating the monumental work with the ponderous awe it usually receives. The choral society's version is fun -- a kind of giant Happy Birthday to the originator of Christmas.

The chorus will act as a guide in a Messiah sing-along Sunday that is expected to fill the Fairfax High School auditorium's 1,200 seats.

"Sellout crowds are very new for us," said the society's president, Mia Cunningham, "and we love it."

Such crowds, society officials believe, are the harvest of intensive work over the last two years. "We hit rock bottom in concert attendance, membership, finances -- everything -- a few years back," Cunningham explained, "and we had to go back and ask some very basic questions."

"Like, what are we really about?" said McCord, who joined the society in 1975, before the crisis. "Is the society here just to get people out of the house, to provide a kind of escape? Should we attract people who are not interested in getting any pressure, who don't want to strive for excellence, who just want to sing?"

These questions started a revitalization effort for the society. The group applied for a grant from the Virginia Commission for the Arts to pay a consultant. After 20 1/2 hours of consultation, some solutions were found.

The society decided it needed to be more aggressive in its publicity and expand its mailing list. But before it could sell itself, Cunningham said, "We had to know exactly what we stood for."

As McCord explained: "Up to this time, the board had been trying to please everyone -- the members of the chorus who were there for fun and those who were serious about their music. Well, you just can't please both."

The group opted for quality, instituting annual auditions, stricter attendance policies and higher singing standards. "We want people who can sight-read music and who don't have any distracting qualities: no sopranos with big, wobbly voices, please," said McCord.

Although a few members dropped out rather than face the auditions, most "loved it," said Cunningham, "and our membership rose from 42 to 97 members in two years."

"We found that we had been attracting quality people," said McCord, "but they got fed up with the lollygagging and dropped out. When we started to get serious about this, they returned."

The group also tried to appeal to a broader audience by offering last spring a concert of light music, based on nostalgic melodies of the operetta. The result was a sellout -- the first. "There were our chorus members, who usually stand with these serious expressions on their faces," said Cunningham, demonstrating, "and for the first time they heard the audience cheering and shouting 'Bravo!' in the middle of a song. It was wonderful."

Audience attendance has gone up 250 percent since the consultant was hired in 1979, she said. More telling to her, though, is the increased corporate support the society is receiving. A $4,800 grant from Mobil is underwriting this Sunday's concert, and contributions from 20 other organizations help make up the society's $35,000 annual budget.

Much of the money, however, still comes from the choral members themselves. "They pay annual dues and buy all their own music: about $40 worth this year," said Cunningham. "They also provide their own uniforms and then go out and help with the fund-raising. This is not a cheap hobby."

Nor is it an easy one. The society rehearses weekly from September until May, with added rehearsals before each of its four concerts. In addition, 15 of the choral members form a chamber ensemble that performs at nursing homes, shopping malls and churches throughout the area.

"It all indicates a commitment," said McCord. "These people are serious about their music and appreciate an atmosphere that treats it seriously."

"Messiah Sing-Along" with the Fairfax Choral Society and concert soloists, Sunday at 4 p.m. in the Fairfax High School auditorium, 3500 Old Lee Highway, Fairfax. Admission: adults, $4; senior citizens and students, $2. Scores will be available for purchase in the lobby. For information and reservations, phone 281-1689.