Lucy Neville Smith, 92, a retired government employe who made her acting debut at the age of 75 in a movie filmed here with Warren Beatty and Jean Seberg, died of cardiac arrest Monday at Shady Grove Adventist Hospital in Rockville.

The movie, "Lilith," filmed here in 1963, marked Miss Smith's only appearance before the cameras. Her short-lived acting career had come about by happenstance.

Movie makers from Columbia Pictures were looking for a rambling, regal residence to use as a set for "Lilith," the story of a sanitarium worker who falls in love with a psychotic and attractive patient.

The movie was based on a book by J.R. Salamanca. The town in the book closely resembled Rockville and probably accounted for Columbia Pictures' suddenly contacting the Rockville Chamber of Commerce for help in locating a suitable site for the film.

The "sanitarium" for the movie was Miss Smith's vintage home at 108 Forest Ave. in Rockville, where she had lived since 1890. She continued to live there with a brother and a niece while the film was being made.

"I told them to make themselves at home," Miss Smith said in a Washington Post interview at the time. She said that the movie people "used the dining room as an office. Oh, they were all over the place. . . I had a time in the kitchen because we still had to eat, don't you know."

She also was pleasently surprised with the actors and crew.

"Not at all what you'd expect, never roughneck," she explained.

Then, her big chance arrived. The movie company came to know Miss Smith and thought her perfect to play the part of Warren Beatty's grandmother in the film.

"We've never done anything like this in the family before. I thought they were kidding," she explained.

She agreed to travel to New York, and filmed two scenes with Beatty, earning $700 for two weeks' work, plus expense money. Included in the expenses were room and board for her niece, who acted as Miss Smith's chaperone.

Miss Smith, a lifelong member of Rockville Presbyterian Church where she had been an organist and pianist as well as Sunday school teacher, did have some nagging doubts about the picture.

She said she had not read the book on which the movie was based. If she had, she later said, "I don't know if I'd let them come up here or not." But her fears were unjustified, she later told a reporter. The movie was "toned down" and "Jean Seberg doesn't strip," she explained.

Miss Smith was born in Washington and reared in Rockville.She graduated from the old Miss Lucy Simpson School in Rockville and attended the Washington College of Music.

She worked for the Veterans Administration from 1916 to 1955, and rose to the post of section chief in the claims department's file division. She also had taught music, piano, and organ at her home over the years.

Miss Smith was a member of the Rockville Garden Club, the Rockville Women's Club, and the Rockville Historical Society, and a charter member of the Kings's Daughters Society, a charitable organization.

Survivors include a brother, Duncan C. Smith of Joliet, Ill.