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We might have guessed from Hugh Grant’s “Psycho” reference in “Four Weddings in a Funeral,” comparing a former girlfriend to Anthony Perkins’ maniac, Norman Bates, that there were darker thoughts beneath that tousled head than fears of marriage.

Indeed there were, as least as far as Grant’s characters are concerned. In recent years he’s been moving away from the rom-com charm that made his fame and toward more unlikeable and unhinged personalities, including the scheming husband of HBO’s “The Undoing,” the narcissistic thespian of “Paddington 2” and the cranky Oompa Loompa of “Wonka.”

Grant reaches an unnerving apotheosis of anti-social behaviour with “Heretic,” a psychological horror film written and directed by Scott Beck and Bryan Wood (the screenwriters of “A Quiet Place”) that turns Grant’s awkward grin into a symbol of fear and dread.

His rictus of a smirk is plastered on the face of Mr. Reed (no first name), a quiet man in nerdish specs and a checked cardigan who answers a knock on the door of his house, which is at the end of a leaf-strewn lane. Carrying a water can, he looks harmless enough.

Two Mormon missionaries, Sister Barnes (Sophie Thatcher) and Sister Paxton (Chloe East) have come calling on a blustery day, to see if Mr. Reed is open to discussing matters of religious belief.

“Are you ready to hear about our Heavenly Father’s plan for you?” Paxton asks him.

Such a stroke of luck! Not only is Mr. Reed ready, he’s willing and eager to talk … and talk … and talk … about all major religions and their ties to popular culture.

He puts down his watering can and beckons the missionaries to come in, pass the “Bless This Mess” sign, for a piece of the blueberry pie they smell baking. He promises that his unseen wife will join them shortly to help serve the fragrant treat; the presence of a woman is a necessary condition for Barnes and Paxton, members of the Church of Jesus Christ of Latter-day Saints, to be inside Mr. Reed’s home.

Mr. Reed has a query of his own for his messianic visitors: “How do you feel about awkward questions?”

So begins a dialogue between a man of assertions and two women of certainty, with the latter feeling increasingly uncertain about their personal safety.

Mr. Reed is obviously informed about the tenets of many religious faiths — he even has the holy texts — but he delights in pointing up perceived inconsistencies and hypocrisies within them. He’s also prone, as all zealots are, to stray into side issues and to find links where none exist.

He gets pushback from the missionaries, especially the more forthright Barnes, who finds his arguments facile and wonders why Mrs. Reed is taking so long. Barnes and Paxton, well played by Thatcher and East, aren’t complete strangers to the guiles of men, as indicated by a discussion about sex earlier in the film.

The brilliance of “Heretic,” at least in its early going, is how it makes our eyes doubt what our minds are anxiously thinking.

Mr. Reed looks like just another awkward Hugh Grant character, the kind who has the propensity to say the wrong thing but nevertheless has a good heart.

Yet our minds flash “danger!” at the growing realization Mr. Reed is in no hurry to have his visitors leave his home. On the contrary, he seems determined to make them stay, as he proposes to put to the test what they really believe and to offer “enlightenment” about a better path.

Horror film fans will note a certain Batesian similarity to the birds of prey models glimpsed in Mr. Reed’s abode, along with nods to “The Silence of the Lambs” and even “Donnie Darko.”

The mood of unease is enhanced by the lowlight cinematography of Chung-hoon Chung, who has brought similar richness to the films of Park Chan-wook, among them “O������Dz�,” “Sympathy for Lady Vengeance” and “The Handmaiden.”

And a word about Mr. Reed’s quirky love of the Hollies’ yearning ballad “The Air That I Breathe,” which he plays for the missionaries and which will never again sound the same to me. Might the filmmakers be mischievously tweaking our senses to think of “Creep,” the Radiohead hit built on the same chord progression?

“Heretic” ultimately doesn’t quite reach the high standards of its classic forebears, as it forsakes logic and embraces conventionality in its rushed third act. But it excels as a showcase for Hugh Grant’s dark side. Long may its shadow stretch.