A TEAM of Washington actors is training hard for Saturday's dramatic face-off against top Toronto thespians. The arena: Trinity Theater in Georgetown. The event: Theatersports, an evening of congenially competitive improvisational acting set up like a sporting event, complete with coin toss, a series of acting challenges and an audience that participates actively, yelling out suggestions for instant sketches and hurling foam-rubber "boo bricks" at the judges if they disagree with a scoring call or penalty.

"People seem to have a bizarre idea of what improv is about," says Gary Jacobs, director of the Sylvia Toone Actors Repertory (STAR), a Washington improv-only theater company that is hosting the Canadian challengers. "The cliche' image is people pretending they are an animal or a piece of fruit or melting ice cream or something. Or else they associate it with doing charades at parties." Part of the problem, Jacobs says, is that too few theater companies do improv where audiences can see it.

Improv theater's best-known practitioners are Chicago's influential Second City troupe (which produced Nichols & May) and the comedy team of Monteith & Rand. But improvisational theater is alive and well in Washington, too: D.C. improv groups include Jacobs' 9-year-old STAR; the six-member Washington Improvisational Theater (WIT), which also will participate in theatersports Saturday; and Living Stage, an offshoot of Arena Stage, which performs primarily for special and underprivileged audiences.

Improv actors must be quick on their feet, able to respond instantly to a preposterous challenge. An evening of theatersports events may include such events as the "Emotional Symphony," in which participants must produce assigned emotional states when pointed to by a conductor; "First Line, Last Line," in which the audience supplies the first line and last line of the scene and the actors have to supply a scene that justifies the two lines of dialogue; and "A Day in the Life," in which an audience member is briefly interviewed about his day, then watches it in "instant replay," embellished by the actors.

So Jacobs, a patent lawyer who talks as fast as he thinks, has been rigorously rehearsing his group, not all of whom are actors, several times a week for Saturday's showdown. "Rehearsed improv" may sound like an oxymoron, but Jacobs says refining skills is the only way to assure improv performances that will be interesting to more than just the participants. "Improv training develops your acting skills, and it also unlocks spontaneity and creativity," says Jacobs, apparently forgetting to add that improv builds strong bodies 12 ways.

Jacobs says improvised theater offers "an excitement that is missing in 'scripted theater.' You're aware that everything is being made up at that moment, we're all on the creative edge, so no one really knows what's going to happen next. If theater is really done well, you should get the feeling that it's happening at that moment, and that's rare."

Theatersports takes place at 8:30 p.m. Saturday at Trinity Theater, 36th and O NW. Tickets are $6.50. The Toronto team will conduct an improv workshop for newcomers 2 to 5 Sunday at the theater. Call 965-4680.

Stage Superstitions: Actress Nancy Robinette says the cast of Lanford Wilson's "The Mound Builders" has developed a new pre-performance ritual. Before every show at the Source Mainstage, the actors buy fortune cookies at the Great Ming restaurant across the street, then read them to each other backstage, often trading them "as if we have control over our destinies." There may be something in this rite -- Robinette remembers that a cookie recently told her "You will be drawn to the glamor of the stage."

Money talks: Arena Stage has received a grant from the Ford Motor Company to support development of "The Rivers and Ravines," a play by Heather McDonald, who recently relocated to Washington. "Rivers," which is under consideration for the coming season, is about how the farm crisis has affected folks in eastern Colorado. After Arena's resident acting company presents a Playlab-staged reading of the work-in-progress (in the Arena, Sunday at 2), they're all off to Colorado Springs for a couple weeks of rest, rehearsal and conversation with natives.

What I Did On My Summer Vacation: Expatriate Washington actor-director Carl Schurr is running the Totem Pole Playhouse in Fayetteville, Pennsylvania. The summer theater, which was founded by Jean Stapleton, employs a handful of Washington performers and designers during D.C. theater's summer dry spell. This year the D.C. crew includes set designer James Fouchard, actress Maggie Winn-Jones and actor Robert Spencer. The current production is the English sex farce "Move Over, Mrs. Markham," which should give you an idea of where "Noises Off" came from. The Totem Pole is less than two hours north of Washington. Call 717/352-2164.

Bulletin Board: On Tuesday, President Reagan will present members of the Hexagon charity revue with the President's Award for Volunteer Action. The troupe's 31st show, "Just Desserts," raised $135,000 for the Higher Achievement Program . . . The fifth annual Sisterfire women's culture festival Saturday and Sunday features such dramatic participants as D.C.'s Human Bridge Theater; Sistren, a Jamaican theater collective; and the acclaimed Spiderwoman Theater, a Native American troupe. Call 234-9308 . . . The new-ish Smallbeer Theater Company has opened "The Little Playwright" at Calvary United Methodist Church, 1459 Columbia Road NW, and offers an unusual discount: 50 percent off tickets for all playwrights and anyone living within five blocks of the theater.