The actress Mary Astor (1906-87) is likely best remembered for her performance as the shifty client of Sam Spade in director John Huston’s adaptation of Dashiel Hammett’s “The Maltese Falcon” (1941). Why shifty? Because this femme fatale came with two noms de plume: Ruth Wonderly and Brigid O’Shaughnessy. The logic behind the ruse is of a piece with this, among the most convoluted of detective yarns. The picture is best savored for the chemistry between its actors and the tart dialogue. Asking too much sense from some things misses the point.

Did the actress at all mind that her co-stars — not only Humphrey Bogart, but Peter Lorre, Sydney Greenstreet, and the forever put upon Elisha Cook Jr. — garnered more attention? Perhaps not. Astor preferred shucking the limelight: Featured players had careers more long-lived than ingénues. With a raft of credits to her name, including winning an Oscar as Best Supporting Actress for a Bette Davis vehicle, “The Great Lie” (1941), Astor’s finest moment was likely her turn as Pat in Fred Zinnemann’s “Act of Violence” (1949), a film soon to be released on Blu-ray by the Warner Archive Collection.

Astor doesn’t show up until past the halfway mark, nor does she spend a lot of time on screen. Still, her character is pivotal to the movie and Astor knew it: She comes close to running away with the thing. Playing a prostitute who is well into middle-age, Astor brings something to the role other than world-weariness: Pat is an opportunist, but she hasn’t been made completely cynical by the wiles of her trade. When she meets up with a man in desperate need of redemption, Pat doesn’t think twice about extending a helping hand. Existential frazzlement and kindness are not mutually incompatible.

“Act of Violence” begins as the sunniest of films and turns into the grubbiest of noirs. The intro hints at dark tidings. We watch a craggy, stiff-legged man walk through the streets of New York City. He enters a brownstone, ascends the stairs, and retrieves a gun from his apartment. Exiting the building, he cuts a threatening figure with his trench coat, purposeful canter, and grimly set visage. The man gets on a bus headed for Los Angeles. Meet Joe Parkson, a wounded war veteran played by Robert Ryan.

Cut to Frank Enley (Van Heflin) with a baby boy on his shoulders and a young wife by his side, Edith (Janet Leigh). They’re attending an event in Frank’s honor: As both contractor and involved citizen, he’s helped to develop affordable housing. Frank is beloved by his community in this comfy California ’burb: He’s a local hero, having served time in a German prisoner of war camp. As the day commences, Frank has a decision to make. Will he go on a fishing trip with his next door neighbor, the avuncular Fred (Harry Antrim), or engage in some impromptu friskiness with Edith? Either way, life is good.

Not for much longer, though. Parkson is doggedly tailing Enley, who is out for retribution. They both served in the same military unit and once counted each other as the best of friends. The time they spent in the POW camp came with a set of compromises and, with those compromises, betrayal and death. The script by Robert L. Richards, based on a story by Collier Young, asks the audience to shift its loyalties as the story progresses and leaves the characters similarly adrift. Moral qualms are the stuff of high drama, and it’s worth reminding ourselves that director Fred Zinneman would tread related ground three years later in “High Noon.”

“Act of Violence” is a flinty entertainment that’s just a scintilla away from being a classic noir. Why that is, is hard to peg. All the main players are in top form and the supporting cast, even when playing to type, deliver the goods. Cinematographer Robert Surtees is inventive throughout — he gets a lot of traction casting aspersions on characters through fast contrasts of light and dark — and Zinneman proves as terse and pitiless as the story itself. 

The plot does wander a bit at times, I suppose, but then so do the demons haunting our protagonists. Whatever: There are worse ways of honing one’s critical faculties than having to re-watch this moody meditation on good intentions and bad decisions.