By day Diane Stanley is a Fairfax County Circuit Court clerk. By night her voice tells more than 500 county residents what they will be doing the next day.

Every evening Stanley record a message for the court that tells prospective jurors who call a special number whether they should show up the following day for possible jury duty.

The recorded messages were begun several months ago in Fairfax to save money and jurors' time. Previously jurors waited around the courthouse most of the day, often to discover that they were not needed to serve on a jury.

Recorded messages for jurors are becoming increasingly popular in the courts, according to many court officials, and Fairfax was a relatively late comer to the system in the Washington area.

Arlington adopted a similar system two years ago, following the lead of the U.S. District Court for the District of Columbia which began using recorded messages four years ago. Alexandria and Fairfax use variations of the system now, too. Local Maryland courts require prospective jurors to call each night, but they are greeted by a live voice that tells the caller whether to appear the next day.

The only Washington-area court system that does not give jurors any advance warning on jury service is the D.C. Superior Court., where hundreds of jurors appear every day for two weeks in dingy, crowded rooms, waiting to be called for a trial. Evan if they are not called they are required to sit around the courthouse all day. The district's system was improved slightly last month, a court clerk said, when jurors were required to report every day for two weeks rather than for month.

In the jurisdictions using the phone message service prospective jurors usually are selected the afternoon before a scheduled trial. The number of jurors needed is determined by the number of jury cases scheduled for that day. The propective jurors names - sometimes they are assigned numbers - are read into a recorder.

Under a system previously in use in Fairfax. Arlington and Alexandria, if jurors were not needed or if a case were dismissed or settled at the last minute, clerks and sheriffs department employees had to try to phone each of the jurors at home to warm them not to report to court.

Often those calls wouldn't get through and jurors would show up at court when they weren't needed. The jurisdictions would have to pay them - usually $12 a day and 15 cents a mile traveling expenses - even if they were only sent back home.

"I used to have to take the list home and call from there," said Alexandria Deputy Sheriff Stanley W. Carskadden. "This," Carskadden said patting the tape recorder is a City Hall cubbyhole, "is my second hand girl Friday."

Alexandria got the incentive to use the plan from Arlington. "I just got a little bug in my head, figured it out and just made the move," Carskadden said of the first tape-recorder he bought for the city on his own. The city is on its second recorder.

Now the city saves between $6,000 and $7,000 a year, he said and the jurors who love it, save a lot of time.