John Cassavetes didn't make it to the United States Film Festival due to an illness, but 15 of his films - as actor and director - are playing throughout the week and a seminar was held Saturday to discuss his work.

One who did come to speak for Cassavetes was his long-time friend Seymour Cassel, a popular Hollywood character actor most recently seen as one of Richard Dreyfuss' comrades in "Tin Men" and a doctor in Nicolas Roeg's "Track 29." Cassel also introduced and discussed some of Cassavetes' films in individual screenings. (Right now he's back in rehearsals with Warren Beatty, Madonna and George C. Scott for the movie version of "Dick Tracy," which starts shooting Jan. 30; Cassel plays Tracy's sidekick Sam Ketchum.)Though he is most often cast in comic or heavy character roles by Hollywood producers, Cassel had lead roles in Cassavetes' "Faces," "Minnie and Moskowitz," "The Killing of a Chinese Bookie" and "Love Streams," all showing at the festival.

The mutual admiration goes back 30 years.

"I'd gone to New York to study acting with the American Theater Wing," Cassel explained, "and John was the hot young actor at that time. He had done every `Playhouse 90,' `Studio One' and `Omnibus' (dramatic TV anthology shows) available. He starred in the movie `Edge of the City' with (Sidney) Poitier. And he had opened this workshop and gave out some scholarships to get it started. I went by and met him, he interviewed me for the workshop and said all the scholarships were gone. But he said I was welcome to study in an acting class if I was interested.

"Then he said he was going to work, that he was shooting a film. I asked if I could watch and we went into the next room and they were shooting `Shadows' (Cassavetes' first directing effort, an extremely low-budget independent experimental film).

"I stayed all night, helping move cameras around and just pitching in. I became associate producer - it just sort of evolved - and had a small part in it. That's where I learned the first things I knew about filmmaking. The actual shooting took 10 weeks, but the post-production took about two years. And because it was an improvisational film we put it together and then had to shoot added scenes."

Cassel says it was the improvisation in "Shadows" that prompts people to assume Cassavetes' other films are also improvised. But he says most of them contained hard scripts that Cassavetes would alter if necessary as shooting progressed. "He'd just disappear into another room for an hour and then come back with a new scene."

" `Shadows' was about 80 percent improvised," Cassel explained, "but the rest of the films were all written, usually as plays. We would rehearse for two weeks before shooting, and two of the films - `Faces' and `Love Streams' - were shot in continuity (in the order scenes are shown in the film; most films are shot out of sequence). He's the only one I've worked with that's done that. It's almost unheard of, but it helps build the character properly, like in the theater."

Cassel has worked with many top directors, and easily ticks off some of his favorites: Elia Kazan, Sam Peckinpah, Nicolas Roeg, Ken Russell. But, he says, Cassavetes is at the head of the list.

"Although I am prejudiced, of course, he's the best. I think he's the most insightful director on the American way of life, relationships people have with each other, whether in marriage or everyday life."

Cassel said that though Cassavetes has been ill, he remains active. Last year Cassavetes directed a play in Los Angeles, and he's always working on scripts. "He's maintaining pretty good. He won't act anymore, but I think he will be able to direct again. I think he can. But I don't know which of the eight or 10 scripts he's working he'll want to do. This man is a workaholic and always has been."

- THURSDAY'S BEST BETS: "Yosemite: The Fate of Heaven," a documentary narrated by Robert Redford and produced by the Sundance Institute, will show Thursday morning, and Redford himself will introduce the afternoon seminar "Latin American Cinema Today," with several Latin filmmakers in attendance who have films in the "Dangerous Loves" section of the festival.

Top documentaries in the independent competition that are showing Thursday include "Let's Get Lost," "Who Killed Vincent Chin," "La Ofrenda" and "For All Mankind." On the dramatic side of the independent competition there are "84 Charlie Mopic," "Apartment Zero" and "Of Men and Angels."

The United States Film Festival is presented by the Sundance Institute, in cooperation with Utah Film Development. Festival officials suggest you phone ahead to see that seats are still available for the movies you want to see and to reserve tickets. The festival information line is 322-1700.

Here's the schedule for the next couple of days:

*****

THURSDAY

Egyptian: "Yosemite," 10 a.m.; "Lobster Man From Mars," 1 p.m.; "84 Charlie Mopic," 4 p.m.; "Dead Man Out," 7 p.m.; "Apartment Zero," 10 p.m.

Holiday I: "Morgan's Cake," 10:30 p.m.; "Who Killed Vincent Chin?" 1:30 p.m.; "Lightning Over Braddock," 4:30 p.m.; "Rogues Gallery," 7:30 p.m.; "La Ofrenda," 10:30 p.m.

Holiday II: "Some Girls," 10:15 a.m.; "Funny," 1:15 p.m.; "Gingerale Afternoon," 4:15 p.m.; "Miracle Mile," 7:15 p.m.; "Gloria," 10:15 p.m.

Holiday III: "Opening Night," 10 a.m.; "That's Adequate," 1 p.m.; "For All Mankind," 4 p.m.; "Let's Get Lost," 7 p.m.; "Of Men and Angels," 10 p.m.

Prospector: "Prisoners of Inertia," 10 a.m.; Seminar: "Latin American Cinema Today," 1 p.m.; "Discovery Program," 4 p.m.; "The Big Dis," 7 p.m.; "Comic Book Confidential," 10 p.m.

Elks Building: Seminar: "Video Presales," 4 p.m.

Sundance: "Yosemite," 7 p.m.

*****

FRIDAY

Egyptian: "Powwow Highway," 10 a.m.; "84 Charlie Mopic," 1 p.m.; "Heathers," 4 p.m.; "Two-Lane Blacktop," 7 p.m.; "Thelonious Monk," 10 p.m.

Holiday I: "Salute to Jay Ward," 10:30 a.m.; "Morgan's Cake," 1:30 p.m.; "Motel," 4:30 p.m.; "La Ofrenda," 7:30 p.m.; "Rogues Gallery," 10:30 p.m.

Holiday II: "The Tempest," 10:15 a.m.; "Apartment Zero," 1:15 p.m.; "Let's Get Lost," 4:15 p.m.; "sex, lies and videotape," 7:15 p.m.; "Clownhouse," 10:15 p.m.

Holiday III: "Letters from the Park," 10 a.m.; "That's Adequate," 1 p.m.; "The Killing of a Chinese Bookie," 4 p.m.; "Of Men and Angels," 7 p.m.; "Shadows," 10 p.m.

Prospector: "Cheap Shots," 10 a.m.; "Lodz Ghetto," 1 p.m.; "True Love," 4 p.m.; "The Big Dis," 7 p.m.; "Heavy Petting," 10 p.m.

Elks Building: Seminar: "Video Presales," 4 p.m.

Sundance: "Miracle Mile," 7 p.m.