Dick Cavett introduces Agnes DeMille, who is his talk-show guest tonight at 10 on Channel 26, as "one of the giants" of American theater, and for once, the phrase is more than hollow press-agentry.

DeMille was one of the first choreographers to let the refreshing breezes of American vernacular dance onto the ballet stage, in works such as her "Rodeo." She also pioneered in bringing the brilliance and emotional force of classical dancing to bear on the levities of musical comedy, in such shows as "Oklahoma," "Carousel" and "Brigadoom."

She has as well a vast list of credits as a choreographer for film and TV, as a stage director, as an author, and as a stateswoman for the arts generally. She is, besides, a woman of great wit and indomitable spirit.

As an introduction to this many-sided artist and personality, the Cavett interview serves fairly well. Looking wonderfully fresh and fit despite the severe stroke she suffered a couple of years ago, the 68-year-old deMille proves as sharply articulate as ever.

She recounts some of the highlights of her often turbulent career, including incidents she has described very engagingly in several autobiographical books. Cavett asks her about the hostility she encountered while staging "Rodeo" among the dancers of old Ballet Russe de Monte Carlo, who were aroused over such unclassical innovations as tap-dancing.

"This is not dancing," they said," deMille recalls. "I said, "Who said it was? Do it." But they didn't want to do it - they couldn't do it. So they stomped out of the studio. I didn't care."

She also talks about the short shrift she received financially for her contributions to "Oklahoma," one of the box-office blockbusters of Broadway history. Spurred by her impending marriage to "a penniless soldier" - this was the early '40s - she had the temerity to ask that her weekly salary of $50 be raised to $75. The Theater Guild's Lawrence Langer told her, "You don't have a legal leg to stand on." "I said to him," she tells Cavett, "I'm not standing.I'm not sitting - I'm on my knees; I'm imploring you." The raise was turned down.

Not many artists would be as freely self-deprecatory as deMille can be about some of her own finest work. After looking at a clip from the movie version of "Oklahoma" she remarks, "It looks like old hat now, it's been so imitated. And those double turns in the air - what American cowboy would do such a thing? I'd never do that now."

Details of her upbringing also surface - the opposition of her father, William, who was a notable playwright and film director, to dancing, which he regarded as "degraded and silly"; the overprotective home atmosphere, in which the reading of daily newspapers was strictly taboo.

Cavett prompts her into telling a corking story involving her celebrated uncle, Cecil B. De Miller: "He once hired you to do a number for 'Cleopatra' in which you were supposed to dance on the back of a bull, is that right?" "Naked," she replies, "and I'm scared of cows, you know."

"The bull was tethered on the Lasky lot," she explains. "I introduced myself, got on his back - he was okay, didn't try to throw me or anything. I found him so much pleasanter than the business manager, or any of the managers on the set. Finally when I was fired - which of course I was - I went back put my arms around him and wept."

The only trouble with the program is the questions that never get asked. One wouldn't expect an interview of this nature to delve very deeply into dance. But deMille is outspoken on a great many subjects - the way the National Endowment for the Arts has spent its dance money in recent years for instance. Even when Cavett gets an opportunity to explore deMille's flintier side, he lets it go. He asks, for instance, whether she saw any of her own choreographic influence in "Chorus LIne." "Not one bar," deMille replies emphatically. "It's not my cup of tea."

This too is a matter on which deMille can wax quite eloquent, but Cavett merely looks miffed that he guessed wrong and hastens on to another questions. Half a deMille, however, is a good deal better than none, so we should be grateful for the interview in the first place.