NEW YORK -- The audience watching "Housekeeping" will never see exactly what Christine Lahti, starring as the enigmatic Sylvie, is carrying in her baggy coat pockets.

She'd half-hoped for a close-up of the stuff. Lahti had carefully collected the detritus that Sylvie sifts through, in a train station scene, to approximate what a rail-riding transient of the 1950s might have accumulated. "There was a compass, a train schedule, rocks I'd polished, little treasures," Lahti remembers. "A sugar packet. Salt and pepper. They didn't have catsup packets in the '50s. Crackers, of course."

Because director Bill Forsyth ruled against zooming in (he was right, Lahti acknowledges wistfully), that particular element of an actor's preparation will go unnoticed. Still, Lahti insists, "it matters to me ... It feeds me."

Ultimately, it matters to anyone who sees, on the screen, a waifish wanderer warming her neck with a newspaper scarf instead of the lanky, copper-haired woman now uncurling into an art deco sofa. The apartment is sun-washed, sophisticated, brightened with posters and lilies, resonating with a taped piano concerto. Lahti, the New York actress, belongs here; the strangely childlike Sylvie doesn't.

Most of the necessary transformation, Lahti says, took place not on location in British Columbia but during the six hurried weeks she had, before filming began in the fall of 1986, to figure out who Sylvie was.

"Housekeeping," which has found its way onto several 10-best lists (and opens today in Washington), practically defines the term "little film." It had the requisite puny budget ($6 million) and long battle for financing (2 1/2 years).

But the stakes were high. Forsyth was shooting his first movie outside Scotland, where he made such admired little films as "Local Hero" and "Gregory's Girl." Lahti, who'd spent much of her career on New York stages and as the star's-best-friend in movies like "Swing Shift," was finally getting "to do what I'd known for years I could do -- play the lead in a movie, a wonderful movie."

And the part, which came her way after Diane Keaton suddenly withdrew, was a confounding one. Marilynne Robinson's 1980 novel, from which Forsyth fashioned the screenplay, is a vaporous memoir written from a child's perspective. Sylvie Fisher simply appears at the family home in Idaho one day to assume care of her two orphaned nieces, Ruthie and Lucille. Ruthie hears her elderly aunts describing the long-absent Sylvie:

"An itinerant."

"A migrant worker."

"A drifter."

The girls discover that Sylvie prefers to eat supper in the dark, occasionally naps on park benches and has no idea what became of her husband. But even after reading the novel, Lahti told Forsyth, she didn't understand a lot about the character.

Both wanted to avoid turning Sylvie into Auntie Mame, "this romanticized free spirit, this sweet, dotty, eccentric," Lahti says. But the script was "like a skeleton, a lot of questions, a lot of holes. I had to do a lot of filling in."

She began at the New York Public Library. "I took out everything I could on hobos, transients, just to learn about the life," she says. She read about the Depression drifters and about Boxcar Bertha, one of the few women hobos to whom she found reference. From her reading Lahti adopted the layers of newspaper Sylvie wears beneath her coat and the lumpy contents of her coat pockets.

In contrast to the theater, "so much of {film acting} becomes reality," Lahti explains. "You're actually in a train station; you don't have to imagine one ... Specific props help."

Observing New York's own contemporary transients proved problematic. Lahti's own Upper West Side neighborhood has its share of homeless people, but she found most of them "disturbed in a way Sylvie was not." She approached a couple of homeless women at Grand Central Station; they turned away.

But the Bronx Zoo, of all places, yielded a gesture. Lahti had gone there in search of an animal that might inspire her and found "a kind of monkey that was absolutely right. It was calmly sitting by the water, eating like this" -- with her fingers, Lahti quickly transfers imaginary food from raised palm to mouth, shoulders hunched protectively. "Not nervous, but very wary." Sylvie eats like that monkey.

And she walks with a gait suggested by Annie Dillard's meditation on nature, "Pilgrim at Tinker Creek," in which Dillard speaks of taking long strides "so as to feel the curve of the earth." Lahti, who'd read the book years before and came back to it, thought Sylvie would walk like that, somewhat defiantly covering ground.

Probably the most important piece of preparation was Sylvie's biography, which Lahti wrote out longhand before leaving for British Columbia. Drawing hints from the novel and inventing the rest, delving into Sylvie's childhood and schooling and marriage, filling in the blanks that Marilynne Robinson and Bill Forsyth had left, Lahti imagined a detailed history -- for her own eyes only -- for a character who barely had one.

Forsyth approved. "It's all right for the rest of us not to know where Sylvie came from," he says by phone from his country house outside Glasgow, "but it's very important for Christine."

All of Lahti's characters -- from the physician in "Whose Life Is It Anyway?" to Goldie Hawn's wisecracking sidekick (for which Lahti won an Oscar nomination) in "Swing Shift" to Maggie (onstage) in "Cat on a Hot Tin Roof" -- have gotten the same treatment, drawn from a writing textbook recommended by a former acting teacher.

Sylvie, Lahti decided, had married young to escape her mother's strict conventionality. Her new husband went off to war and Sylvie "worked in an airplane factory and had a taste of freedom for the first time." (Why waste all the research she'd done on Rosie-the-Riveter types for "Swing Shift"? Lahti reasoned.) When he came marching home, he found Sylvie had become too independent for his tastes. "He couldn't tolerate it, so he left her. I think it really devastated her -- as did her father's death when she was 13. That was the major trauma of her life."

By the time Lahti and the rest of the cast began two weeks of rehearsals, Forsyth recalls, "she was Sylvie." At the same time Lahti noticed that Forsyth -- "who is eccentric and completely lacking any vanity" -- was apt to come to work with his cardigan misbuttoned. She appropriated that detail, too.

Sylvie's clothes are carefully nonglamorous. The costume designer had given her a brown tweed skirt that fit. "It looked pretty great," Lahti remembers. "And I thought, no, it should be too big." She added large safety pins at the waist, "so it bunched and gapped and looked like something I found." Every morning the wardrobe people sprayed Lahti's costumes with water and sat on them to achieve the desired wrinkling. (Sylvie doesn't iron.) Every morning, too, Lahti went outside the set and rubbed dirt under her fingernails.

During nine six-day weeks of shooting, she found herself withdrawing, Sylvie-like. "I became much more alone. Private. I lived in a log cabin up in the mountains. I hardly socialized."

The character that emerges from the zoo visits and the pinned-up skirt, from Forsyth's direction and Lahti's training (primarily with Uta Hagen and William Esper of New York's Neighborhood Playhouse) is at least as troubling as she is charming. Audiences have been audibly ambivalent about whether Sylvie should or shouldn't be mothering Ruthie, about whether the final scenes in "Housekeeping" are a triumph or a prelude to disaster.

"Yeah, that's the desired effect," grins the star.

"That's fine by me," agrees the director. "I'm not crazy about happy endings. I don't think a lot of lives have more than finite moments of happiness in them, so why should movies?"

Lahti was not the only knockout playing a lowlife this year, of course. But -- here the discussion degenerates into gossipy girl talk -- Faye Dunaway, an alcoholic in "Barfly," still flashed those architectural cheekbones; Barbra Streisand, a mental patient in "Nuts," somehow must have had time between therapy sessions to streak her hair. "I didn't believe her for one minute," Lahti says.

But "what about Meryl?" she wonders. She hasn't seen "Ironweed" yet -- did Streep relinquish flattering lighting and makeup for true beat-up homeliness as an Albany derelict? "If I'm playing a real glamorous lady I love to be lit well," Lahti sighs. "But not for Sylvie."

One of the questions hovering around "Housekeeping" is whether it will help turn her into "Christine" -- one of those no-last-name-necessary actors with the clout to be the masters of their fate. If it makes money -- if it does as well as "Local Hero," say -- it may make a difference, Lahti figures. If it doesn't make money, it won't.

"I'm not a major movie star, obviously, and that's all right," she reflects. "I make a living; I don't need to make $6 million a picture."

Though it's true that this is her first cinematic leading role -- she's 37 -- Lahti no longer has to audition for soap operas (she tried them all and never landed a part); wait tables (she hustled tips from Village bars to Madison Avenue lunch spots); or do commercials (she hawked Sominex and Spray 'n Vac and admired her reflection in a dish washed with Joy).

In fact, Lahti says, she recently turned down "an extraordinary amount of money" to push Maxwell House coffee. "Even my husband" -- television director Thomas Schlamme -- "said, 'Oh, do it. Who cares?' I just couldn't." Commercials "really did bail me out, so I'm grateful, but I always felt a little dirty."

So she's reasonably secure and about to begin rehearsing "Summer and Smoke" on stage in Los Angeles, with Christopher Reeve as her costar and Marshall Mason directing -- a plum. Still, she can't deny the appeal of possibly, eventually, becoming a hot commodity.

"I would like the opportunities a Movie Star has," Lahti admits. "Like getting offered the best scripts. Right now they go to Jessica Lange and Meryl. And Cher, Cher's the flavor of the month. I get to see them if they pass. I'm second string."

It is therefore not true, Lahti says, that she is sensitive about not being Forsyth's first pick for "Housekeeping." Not at all. After years of being rich in respect and low on the marquee, too tall or too unknown or too something to carry a movie, she's a star.

"I was only thrilled that Diane Keaton chose not to do it," she insists. "I felt, 'Awwright! This is great!'"