'Angelo My Love' is brilliant, compassionate; It all started when Robert Duvall spotted a gypsy boy . . .

The movies have a middle ground that is rarely explored - the twilight zone between fiction and documentary. But occasionally a filmmaker comes along who's too inventive, audacious, or plain restless to respect the usual boundary lines.

One is Robert Duvall - yes, the same Robert Duvall whose performance in ''Tender Mercies'' is one of the most finely wrought achievements to reach the screen in recent memory.

Not content with being only an actor, he launched a directing career a few years ago with a documentary called ''We're Not the Jet Set,'' a memorable study of a ranching family.

Now, working again behind the camera, he has created a small miracle called Angelo My Love - a colorful, witty, and relentlessly dramatic picture that boldly mixes fiction and truth in a strikingly unusual approach to capturing the human comedy on film.

It all began when Duvall was strolling down the street in his Manhattan neighborhood and spotted a young gypsy - about eight or nine years old - whom he had noticed before. The boy was talking earnestly with a woman in her late 20s, and as Duvall came closer, he heard the lad's parting words: ''If you don't love me no more, Patricia, I'm gonna move to Cincinnati!''

It's a story Duvall loves to tell, and it is now being used in advertisements to promote ''Angelo My Love,'' the movie sparked by this chance encounter. Struck by the boy's precocity, charm, and streetwise intelligence, Duvall befriended him and got to know his family. Soon he had a circle of gypsy acquaintances he was eager to catch on film. But what method should he use? Would documentary or fiction best convey the essence of these fascinating individuals?

Casting about for the answer, Duvall turned screenwriter and began work on a script about two families feuding over a stolen ring. The story was fictional, but he carefully included roles that would echo the personalities, looks, and talents of his new acquaintances - many of whom jumped at the chance to play themselves in the movie.

Duvall then shot the picture over a 10-week period, altering the plot and characters to suit changing conditions: incorporating preparations for an actual wedding, for example, or eliminating a character when a gypsy performer abruptly left town.

The result is fiction illuminated by the glow of truth. In real life, there was no stolen ring or feud between Greek and Russian clans, as fabricated by storyteller Duvall. But there truly is a boy of exuberant charm named Angelo Evans, and his brother did ''buy'' a bride before marrying, and his family and friends do share the kinds of joys, miseries, aspirations, and superstitions we see here - depicted so vividly that some aspects (including occasional harsh language and dissolute behavior) might have mandated an R tag if the independently produced film had passed through the rating process.

As I watched ''Angelo My Love,'' parts of it felt almost like a science-fiction movie, so removed are the characters and their folkways from the paths I usually travel. Yet the film's compassion is clear in every scene; never does Duvall sensationalize or condescend to his material, although he often treats it with enormous humor.

I talked with Duvall about the film in his comfortable Manhattan apartment a few days ago. He was notably relaxed and good-humored, and seemed thoroughly satisfied with ''Angelo,'' which - taking an unusual risk - he financed out of his own pocket. He is aware it's a ''special'' movie that won't easily rack up the box office grosses of an outer-space or adventure epic. But it clearly reflects his vision of the gypsy community that so captivates him, and this makes it a rousing success as far as he's concerned.

Complimented on the sense of authenticity that runs through the picture, Duvall says there was no need to beef up or Hollywoodize his material, since his performers - all natural actors in a ''con artist'' sort of way - needed little encouragement to play their parts to the hilt. In fact, the director says, they brought in sly nuances and subtleties that no outsider could have invented. And, though amateurs, they had the knack of repeating scenes for the camera as readily as the seasoned professionals Duvall has worked with in Hollywood films.

''They were already actors by necessity,'' Duvall says. ''It's an ability they've picked up in daily life. It comes from having eight children, but needing to convince a landlord you only have two, so he'll rent you a storefront you want - and going through a hundred other trials like that, all the time. Fooling people is a part of living for them.''

The brilliance of ''Angelo'' stems partly from Duvall's delicately balanced attitude toward his unconventional collaborators - never ignoring their sometimes drastic flaws and foibles, yet keeping sight of the inner dignity and emotional vulnerability that are also essential parts of their natures.

The filming of ''Angelo'' had plenty of difficulties, according to Duvall and his wife, Gail Youngs, who served as associate producer. Operating with a vague sense of time, gypsy cast members thought nothing of showing up three hours late for a ''shoot,'' keeping dozens of extras and crew members waiting. Since most of the performers were illiterate, dialogue had to be improvised from lines supplied (sometimes in an off-camera whisper) by Duvall.

And finances had to be watched closely, since the director was paying the bills with no help from studio sources. ''I'm still catching up, taking roles I'd normally leave alone, working to replenish the till,'' he says, with the bemused tone of an artist who's not accustomed to spending on such a scale, but knows both time and money have been well used.

''The movie grew from his love for these people,'' says producer Youngs, underlining the affection that drew Duvall to the subject and kept him immersed in it for several years. Duvall himself seems most proud of the authentic behavior the movie depicts. In fact, he feels his main strength as a director - building on his long acting experience - is a skill in guiding performances and bringing out their nuances.

''Angelo My Love'' won't be the last Duvall-directed film. More and more, he feels the urge to develop projects himself, making worthwhile things happen instead of waiting for them to drop in from outside. He still expects to play plenty of roles in other people's movies - in fact, he says, good parts are coming his way in unusual quantities nowadays - but his filmmaking career is also here to stay. Judging from the strength and substance of ''Angelo My Love, '' that's mighty good news for the movie scene. Overlooked works

The influential Collective for Living Cinema is launching a new series called ''Not Forgotten,'' focusing on filmmakers who are no longer living, and whose contributions may therefore be overlooked or undervalued. The first installment comes tomorrow night with a rare screening of ''Christmas on Earth,'' by Barbara Rubin, a 1964 work that caused a stir when new because of its clinical (and sometimes perverse) nude footage. Today it seems of interest for its dense nonnarrative structure and striking use of complicated superimpositions, and the fact that different projection arrangements can be used when showing it - projecting it normally, for example, or projecting one reel on top of another to create strong new visual textures.

The program continues on May 13, with 8-mm works by Greg Sharits (proceeds will go toward restoring his films), and June 5, with the entire oeuvre of Chris MacLaine. Whatever the worth of the individual works involved, it's good to have an organization concerned with preserving films whose noncommercial nature often keeps them off the screen even when their makers are around to push and promote them. Hitchcock gems

Great news from Hollywood! Universal Pictures has acquired worldwide rights to five films by Alfred Hitchcock - movies owned by the Hitchcock estate, and long held back from distribution in hopes of boosting their box office value.

This means moviegoers could soon have a chance to resee some of Hitchcock's greatest works, including ''Vertigo,'' which stands among the finest American movies ever made. Also in the masterpiece category are ''Rear Window'' and - less great, but still a classic - the 1956 remake of ''The Man Who Knew Too Much.''

Perhaps the most exciting entry, because of its rarity, is ''Rope.'' Regarded by many as Hitchcock's most technically audacious achievement, it's an unconventional mystery filmed entirely in 10-minute shots. Perhaps because of its peculiar style, it has been largely unseen since its original release. The final item on the list is ''The Trouble With Harry,'' a minor but engaging dark comedy.

Since all the above (except ''Harry'') feature James Stewart, their reissue will be a boon to his fans as well as admirers of the ''master of suspense,'' who realized many of his most extraordinary works during the period of these pictures, which span the decade from 1948 to '58. Here's one Hitchcock devotee hoping Universal gets these goodies back on the screen mighty fast. Soviet film

The latest Soviet film to visit American screens, Waiting for Gavrilov, is full of ideas - about love, marriage, and movies - that parade just as endlessly through Western films.

The plot, about a woman mooning over her missing fiance, is standard 1940 s-vintage Hollywood. So are the easy-come, easy-go moods. The style recalls the glossy gimmicks of Claude Lelouch and ''A Man and a Woman.'' The characters are as universal as they are ordinary, even if they do speak Russian and stand in line to buy tacky consumer products that wouldn't attract a second glance from their American counterparts.

The director, Pyotr Todorovsky, gives a redeeming charm to some scenes, but he fails to make the package look fresh. In sum, it's a diverting but unexciting example of cinematic detente. Despite its intermittent pleasures, you'll be glad it runs - the opposite of most Russian productions! - not much more than an hour.