ABC publicity is quick to point out that "The Shadow Box," its Sunday Night Movie tomorrow at 9 on Channel 7, isn't "depressing." Not depressing? A play about three terminally ill people awaiting the grimmest of reaps? It's the Winter Olympics of Depression, at the very least.

But Michal Cristofer's play, which won a Pulitzer Prize and a Tony Award, does go far beyond the merely morbid. As handled by Paul Newman, making his TV directorial debut, it's a two-hour ride on an emotional roller coaster that goes mostly down, but occasionally up, and may be an especially appropriate diversion for the post-holiday blue period. This is an unusually clear and unblinking look at the dark at the end of the tunnel.

Obviously Newman learned a great deal about the camera during all the years he's spent in front of it. People will call "Shadow Box" an actor's movie because Newman holds the camera on the actors for long speeches, and each principal gets his or her big wowser soliloquy. But it's an audience's movie as well, and that's what counts.

Many TV movies have dealt with death and terminal illness, but usually just in an unsparing effort to jerk a lot of tears. These films tend to be highly transparent pick-me-ups; death is reduced to the role of a mustachioed villain on a kiddie show. "The Shadow Box" really is about facing death and facing up to it.

And this means it isn't just morbid or gloomy. At times, it's pretty funny. One of those dying is a witty and acebric author played by Christopher Plummer, who says he's on a prolific writing jag because "there's a huge market for dying people right now; my agent assured me." When Sylvia Sidney, as an elderly woman who is dying, hears her daughter singing a hymn, she snaps, "The time for hymns is when I'm in the coffin," and launches into a racy ballad.

The three dying characters and their attendant death-watchers do not interact, but all are staying at a hospice for the terminally ill, away from a cold hospital environment. Joanne Woodward plays a party-prone wife in a singularly precarious position: She has lost her husband to a man and has now come to visit the two of them at the hospice.

"So how is he?" she asks her husband's boyfriend. The young man replies, "He's dying, how are you?"

Meanwhile, Valerie Harper, as the wife of a dying husband played by James Broderick, has arrived from the East stymied and dumbfounded, unable to cope, chattering wildly and laden with miscellaneous bric-a-brac. Later, during a turbulent row on the terrace, Broderick says to her, "You want magic to happen, is that what you want?"

She can't be blamed for wanting that, and the film doesn't blame her. People probably can't be blamed, either, for turning on a television set and wanting magic to happen, wanting to see something more inclined to make a person forget about death than think about it. It's not hard to imagine a full-time TV viewer asking, "Why should I watch a movie about THAT?" It's not as if it's a civic duty, no matter how illustrious the names in the credits.

But in discussing death, wrestling with it and kicking it around, the film also of course talks about life, and the essence of it and the fact that part of that essence is that it ends. Among the many qualities to be embraced in this cheeringly good play is that it gets you thinking about what is remarkable about people you love or just like a lot.

Newman has been generous to, and generous with, all the actors in the film, including his wife Woodward, touchingly fragile in a blowsy whoopee way. Melinda Dillon, as Sidney's long-suffering daughter, is very affecting. The daughter comes to appreciate all the implications, encouraging and discouraging, embodied in words like "hanging on" and "lingering."

Some scenes do not play well, and the character of the husband's boyfriend seems a bit nervously conceived. But Newman certainly shows prowess at both sustaining and modulating moods and in being able to observe the kind of little details that fill out a script and bring it to life. It's probably a more satisfying directorial job than Robert Redford managed with his overpraised and peevishly self-destructive "Ordinary People."

Director Newman scored a quiet little victory offscreen as well. It took a special dispensation form ABC's affiliates to permit the film to run three minutes beyond the usually rigid two-hour air time. This means the stations will have to delay their precious late-night newscasts after the movie. This may sound like a trifling inconvenience, but to them it is one of those money matters that oil the lamp of television.

ABC will preface the film with the by-now-customary parental advisory, this one warning of "explicit dialogue." Undoubtedly the language in the play was considerably more colorful, but this is still a television program that is "adult" and "mature" in the true sense of those words.

"You musn't take all this too seriously," Plummer cautions a doctor at one point."I don't. Our dreams are beautiful. Our fate is sad. But day by day, generally, it's pretty funny." This is a splendid film of a very good play about the biggest punch line of them all.