A third of the way through the production at Hollywood's CAST Theater tonight, the lights will black out for five seconds. When they come back up, the tableau of actors will freeze, and the artistic director will walk on stage and say: "You have just seen as much of this play as your ticket pays for."

The blackout is part of a nationwide mobilization of artists and arts supporters to halt proposed cuts to the National Endowment for the Arts. Other actions include rallies and the handing out of leaflets, petition drives and, in New York, two parades ending at Lincoln Center with an all-star cast. In some cities, the protest have been going on throughout the week.

The movement started in New York, where a group of artists met with the Volunteer Lawyers for the Arts to discuss a class-action suit after President Reagan proposed substantial recisions in the current NEA budget. Reagan subsequently reinstated most of the funds, but "we decided to use that impetus to form a broad-based coalition," said Harvey Seifter, executive director of the American Coalition to Support the Arts. "We called around and we had some success." The result is a day of arts consciousness-raising activities slated for New York, Los Angeles, Boston, Miami, Atlanta, Seattle and San Franciso today.

In Chicago, action got under way a day earlier, as a group of artists held up a funeral shroud at the base of the giant Picasso statue in Daley Plaza. "It was visually marvelous," said Ruth Ganchiff, director of promotion for the League of Chicago Theaters. "All three local TV stations showed up."

In Los Angeles, the past week and this weekend will be a "rolling blackout of theaters across the city," said Susan Dietz, artistic director of the Los Angeles Stage Company. Among the speakers who have delivered speeches supporting the arts at the theaters are Gordon Davidson of the Mark Taper Forum and Lynn Redgrave, who is co-producing a show called "Thursday's Girls."

At some of the theaters, fact sheets and postcards will be passed out to the audience, so theatergoers can dash off a quick postcard to their congressmen during intermission."We expect 25,000 postcards from Los Angeles by the end of the summer," said Dietz, who has been working with a group of volunteers. "Our goal is to get as many postcards as possible to Washington -- from members of audiences, not from artists. They've had their shot."

There will be no rally in Los Angles, but there will be information booths set up throughout the city.

In New York today, Seifter is organizing two parades: One starts from the Theatre for the New City in the East Village at noon and the other at 92nd and Broadway on the Upper West Side at 1:30. They will converge at Lincoln Center for a 3 o'clock rally, hosted by actor Tony Randall. Entertainment will be provided by Roberta Peters, Theodore Bikel, Ossie Davis, Ruby Dee, Michael Moriarity and, from the "Mostly Mozart" festival, pianist Emmanuel Ax and voilinist Young Uck Kim. Speakers include Rep. Fred Richmond (D-N.Y.) -- who formed an arts caucus this winter -- and the presidents of several labor unions. Actress Kim Hunter will read a statement from the AFL-CIO.

The intention, said Seifter, is to "focus public attention and awareness on NEA's role in fostering the quality, diversity and distribution of American art. We expect to generate 500,000 letters to congressmen."

President Reagan proposed funding at the level of $88 million for the National Endowment for the Arts -- half of what the Carter administration had proposed for fiscal 1982. "A 10-percent cut is what we think is fair," said Dietz.

"We know when artists spoke up three or four months ago before Congress, it helped," she added, referring to the fact that House committees have recommended a higher ceiling and appropriation for NEA. "We want to at least go down fighting."