The line that separates life and fictionalization is also the subject of the formidable “Anatomy of a Fall,” directed by Justine Triet (“Sibyl“). The title is clearly a nod to Otto Preminger’s “Anatomy of a Murder” (1959), perhaps the greatest of all courtroom thrillers, and inside a courtroom is where “Fall” spends much of its two-and-a-half-hour running time.

Sandra (Sandra Hüller) and Samuel (Samuel Theis) are a literary couple living in Samuel’s hometown in the French Alps. But when their son, Daniel (Milo Machado Graner), returns from a walk, he finds Samuel’s dead, bloody body in the snow—it appears that his father has fallen from the attic. There are three possibilities: The fall was a total accident. Samuel killed himself. Or Sandra murdered him.

It’s for that last possibility that Sandra is charged. I’m not sure what the standard for convicting someone of homicide in France is, but if I were a juror, I would reach reasonable doubt on the physical evidence alone, and I choose to believe that’s part of the point. A staggering—even exasperating—amount of the trial is spent trying to catch Sandra in lies about circumstantial evidence: a bruise on her arm, a fight she had with her husband, whether her writing foreshadowed the alleged crime, and so on. As a defendant, Sandra (and Hüller, performing in both English and French) makes for a much cooler customer than Pierre Goldman in “The Goldman Case,” a French legal thriller shown here last week.

The key to the film is that it is not, in fact, an anatomy of a murder. It’s an anatomy of a marriage, or specifically how a marriage has fallen apart. The trial functions as fact-finding mechanism both for the court and for Daniel, who begins to understand his parents in a new way, or at least to understand—like Elizabeth in “May December”—how hard it is to understand. This is a dense, talky film with a complicated structure. Aspects that seemed superficial or implausible in the moment have only grown in my mind since it ended.