FOR those who lament the scarcity of mature themes, complex characters, and expressive writing in today's movies, the career of Horton Foote is always a cheering thought.After making his mark in Broadway theater and live television during the 1940s and '50s, he turned to film and earned his first Academy Award for "To Kill a Mockingbird" in 1963. He won a second Oscar for "Tender Mercies" two decades later, and a nomination for "The Trip to Bountiful" in 1985. His work has been distinguished by humanity, compassion, and a sense of deep concern for his plain-living characters. Some years ago, Mr. Foote took a long hiatus from screenwriting and completed "The Orphan's Home," a cycle of nine stage plays based loosely on his own family's history. His newest film, "Convicts," is based on the second of those plays - taking place several years before "Nineteen-Eighteen" and "On Valentine's Day," parts of the cycle that have already been adapted and released as theatrical movies. "Convicts" is set in Texas just after the turn of the century. The main character is Horace Robedaux, a 13-year-old boy living under new and peculiar circumstances - sent by his widowed mother to live on the plantation of his uncle, Soll Gautier, an irascible old codger whose mind is addled by age, alcohol, and eccentricity. Horace has little idea of how to cope with his bizarre relative, or with the convicts who labor on Soll's property. But fortunately he's not alone on the estate. The local population includes the Johnsons, upright African-Americans who run the plantation store where Horace works, and Soll's niece Asa, whose life consists mainly of drinking and wondering when Soll will die so she can inherit his property. Death is on everybody's mind during Christmas Eve. Convinced that his end is near, Soll orders his workers to build him a coffin. Asa is cheerleading for his demise, and even Horace is preoccupied with gathering money to buy a tombstone for his late father. Foote is among the most life-affirming of dramatists, though, and it's no surprise that he finds much to appreciate and even celebrate in these potentially morbid circumstances. "Convicts" is less the story of old Soll than of young Horace, whose resilience and spirit provide the movie's most important driving force. It's also the story of Ben and Martha, and of a convict named Jackson, who shows great fortitude in a white man's world. "Convicts" comes to the screen with an expert cast. Lucas Haas gives Horace impressive depth; James Earl Jones and Starletta DuPois play Ben and Martha with great subtlety and strength; and Mel Winkler is downright astonishing as Jackson, turning a secondary character into the film's most watchable figure. Robert Duvall, continuing an association with Foote that includes "To Kill a Mockingbird" and "Tender Mercies," deserves more measured praise. His portrayal of Soll is a tour de force, but this turns paradoxically into a problem. In his greatest performances, Mr. Duvall shows an uncanny ability to put behavior on the screen without revealing the tricks and techniques behind it. On some occasions, though, his acting takes on a life of its own, and shows through the illusion of true-life behavior. That's what happens in "Convicts," where Duvall employs a too-large repertoire of tics and eccentricities - grunts, twitches, and the like - that calls attention to itself and detracts from the veracity of his work. It's a vigorous perfor mance, but it doesn't stand with the best of Duvall's career. Foote's screenplay also has problems. The movie's interest lies mostly in the relationship between Soll and Horace, which isn't quite rich enough to sustain the entire film. And there's no need for their Christmas Eve encounter to have such a long buildup, paving the way for a diversified story that never materializes. Foote's finest films, such as "Tender Mercies" and "On Valentine's Day," have a structural brilliance that matches the insightfulness of their dialogue and characterizations. "Convicts" isn't quite up to this level, but it's still refreshing in a year when human-scaled originality has made all too few appearances at the movies.