Carl Keener talks fast, plays hard and straddles an invisible horse when his bluegrass band, Willow Creek, goes on stage.

As the bandleader, Keener is "a good psychiatrist and a good whipcracker," says Andy Anderson, who plays bass to Keener's rhythm and sings tenor to Keener's lead.

All five members of Willow Creek are from Northern Virginia. By day, they hold down jobs that bring steady incomes; by night, they play bluegrass to their heart's content.

Anderson is a construction company superintendent. Then there is John Steel, an X-ray technician who runs through "Orange Blossom Special" a couple of times on his fiddle at each performance. And there's a banjo player, a left-handed, mustached computer specialist named Bob Miller, who hunches over his instrument protectively when the picking gets going.

Then there's the mandolin player, Fred Nelson, who grinds his teeth and sets his jaw with each solo, but loosens up when the jokes start flowing. He is an agricultural economist at the Department of Agriculture.

"Fred's kind of tired tonight," Keener informs the audience, a hard-applauding group at Arlington's free concert series at Lubber Run Amphitheatre. "Didn't get much sleep last night," he says, to anticipatory titters. "There were a bunch of women banging on his door, and about three o'clock, he had to get up and let them out."

Keener, who works at the Justice Department, steers quickly into another song, one of 25 played in a 1 1/2-hour performance at Lubber Run. None of Willow Creek's material is original, and much of it, like the band itself, is deliberately crowd-pleasing. The band caters to its public in a big way, and from the moment band members walk on stage, they are deluged with scraps of paper reading "Foggy Mountain Breakdown," "Rocky Top," and "Salty Dog"; the hecklers shout "Orange Blossom Special" after every song.

The band is willing to oblige. "We like an audience," says banjo player Miller. "We'll play forever for a good audience, even if it's only one person."

Their audiences--at parks, picnics, private parties and a club in Occoquan called Talk of the Town--tend to be a lot larger, and louder. Willow Creek has had steady work since Keener and Miller found each other in 1976, despite fierce competition in the bluegrass field ("Washington is the bluegrass capital of the world," says Keener). The reason for their popularity, says Keener, is their price: "If you give it away, there's a big demand." Keener admits the band does charge for performances, but none of the members is trying to make a living at music, a fact that takes the pressure off pushing for fat salaries or regular gigs.

"For years, we wouldn't do clubs, because we didn't want to leave our families," says Keener, who says the band still chooses its audiences carefully, looking for places and times they can bring the kids. "A park is designed for the children," he tells the Arlington crowd, when a couple of blond tots come up to lean on the stage. "We're just filler."

The fillers are songs from Keener's childhood in Gassaway, W. Va., ("I grew up on this music"), and from the wealth of traditional and contemporary bluegrass material taken from albums and tapes. "It's so far going back, we don't have to write anything new," says Steel.

Tunes like "Black Mountain Rag," "Turkey in the Straw" and "Home, Sweet Home" fill up their repertoire of 150 songs. "We have more, but I won't do a song on stage unless we're all comfortable with it," says Keener.

Then there are modern works, like a piece they do from those well-known bluegrass songwriters, the Beatles. "A lot of people don't realize that the Beatles wrote 'Fox on the Run,' " Keener says with a grin. "We also do some Country Gentlemen and Seldom Scene stuff--they're about the best groups in the area."

Again comes the inevitable request for "Orange Blossom Special," and Keener promises it will be the band's last piece of the night, "to keep you all staying till the end." A heckler shouts he got there late and wants another hour of playing. The band, which does not so much milk as dredge the audience, stays another 30 minutes. "If you keep this up," says Keener, relishing every clap, "we will stay here till our wives think we have deserted them, and we are late for work."

Then, while Fred tunes up ("If he ever gets that thing right, we're going to weld it there," declares Anderson), John Steel prepares to convert his fiddle to a whining train, and the Orange Blossom Special takes off.

Willow Creek will give its next public performance at 7:45 p.m. tomorrow at Fort Ward Park in Alexandria. Admission is free. The band also will participate in the birthday celebration for America and Alexandria, which takes place from 10 a.m. to 5 p.m. Saturday at Market Square in Alexandria.