I'm sorry to do this to you, but I want you to picture the face of former US presidential hopeful Ted Cruz. Got it? Good. That lurching in the pit of your stomach isn't an ungenerous reaction on your part, but the result of eight million years of evolution.
Earlier this year, neurologist Richard Cytowic wrote in an essay for Psychology Today that Homo sapiens is hardwired to experience revulsion when Cruz's face looms into view. It apparently all comes down to a mismatch in muscle activity: his zygomaticus majors, at the corners of his mouth, and orbicularis oculis, around his eyes, are out of sync, which is why, whenever he smiles, you can't shake the suspicion that he's thinking about eating your young.
Cruz could have run the greatest campaign in the history of politics, but that leer is unspinnable. It's one of the reasons the internet meme "identifying" Cruz as the notorious serial killer The Zodiac found such a receptive audience. Factually, it's untenable. Physiognomically, it seems about right.
If a face can wreak havoc in politics imagine what one can do in show business. When the trailer for Bridget Jones's Baby was released, Variety ran a screed by Owen Gleiberman, its chief film critic, lamenting that the film's star, Renee Zellweger, "no longer looks like herself".
Gleiberman contended that the actress' noticeably changed face - now leaner and more sculpted than it was in the rosy flush of her early-noughties stardom - was a sad blow "for the notion that each and every one of us is beautiful in just the way God made us" (oh no!), plus some kind of non-specific betrayal of the Bridget Jones character she first played 16 years ago.
In a lengthy (albeit cautiously worded) rebuttal, Zellweger stated she hadn't "made a decision to alter my face and have surgery on my eyes", while bewailing the wider media culture that makes an actress' appearance fair game for comment.
I hold no brief for Gleiberman, whose piece I thought read less like criticism than a closely argued, 1500-word version of the old "actually, I prefer the natural look!" chat-up routine. But there's no sense in denying his underlying point: that film stars' faces have a hold on us that's irrational, unreasonable and not all that easily explicable.
As with Cruz, it's caveman stuff. The same pre-linguistic facial cues our ancestors relied on for survival are what send shivers scuttling down our spines when we watch Jack Nicholson stagger through that blizzard-blown maze in The Shining. His face kindles terror in milliseconds - partly through performance, but even more so through the particular, inimitable hunch of his eyebrows and curl of his lip.
No amount of rehearsal could make that scene work with, say, Tom Hanks or Matt Damon.
Nicholson's expression - head angled downwards, eyes pointing beadily forward, mouth drawn into a thin half-smile - even has a nickname, the Kubrick Stare. And an actor's ability to do it owes nothing to talent or training and everything to whether their facial apparatus measures up.
Nicholson's obviously does. So does Malcolm McDowell's, whose Kubrick Stare as Alex DeLarge in A Clockwork Orange was so blood-congealing that director Stanley Kubrick insisted it went on the poster. Heath Ledger's Joker does it in The Dark Knight, as does Anthony Perkins in Psycho. Kubrick used actors whose faces caught the coiled menace of his films' world-view - just as the Spielberg Face of open-hearted wonderment sums up the mood of Jurassic Park and E.T., or the Coen Brothers' Squint - as regularly practised by George Clooney, Frances McDormand and John Turturro - speaks of the peculiar, neuron-strangling purgatory of living inside a Coen Brothers' film.
Not all of this comes down to an accident of birth. Greta Garbo's face, lineless and pure, sexual and sad, was almost like beauty in the abstract - Roland Barthes droolingly called it "a sort of Platonic Idea of the human creature" - but it could be further refined with makeup and light.
When he was directing Garbo in the final scene of 1933's Queen Christina, Rouben Mamoulian reportedly told the actress to make her face as still as a death mask, then shot a minute of pure expressionlessness. (In the end, he used 40 seconds of it.) The idea was that every audience member could etch their own feelings and fears on to this perfectly blank slate.
The shot starts medium-wide but tracks in, to a classic Hollywood close-up, and it required the creation of a special lens that could manage both. Background details melt away until the face becomes the only thing that matters.
The Coens often get the opposite effect using the opposite kind of lens: wide-angled monsters that cram as much grounding background into the frame as possible.
Donald Sutherland once told me he hates being shot on a lens any wider than 35mm as it "really screws me up". (The smaller the number the wider the lens, and the less flattering it makes faces look at close quarters.) On Barton Fink, the Coens wanted to shoot close-ups of Turturro on the kind of 16mm lens a more reasonable person might use for a panorama of Brighton seafront. Only after a plea for clemency from their cinematographer, Roger Deakins, did they relent and use a 27mm.
But that's the Coens for you. A Queen Christina-calibre close-up - one idealised face, meticulously framed and lit, projected big (TV won't do here), and with a sense that time's passing around it (in this case, Garbo's hair blowing in the wind) - has a particular power that can be hard to unpick.
In Fame in the 20th Century, Clive James makes the best go of it I've read. For him, cinema's face-worship is both sexual and devotional.
We onlookers experience "a greater intimacy with the looked-on than could be achieved by any lover, who would always have his imagination kept in check by the actual texture of a living thing."
In other words, while faces in films look like portraiture, our minds roam around them like landscapes, and part of the thrill is feeling roamed-around in turn.
"You know these people so well," James goes on, "that you can't believe they don't know you."
That explains the seam of entitlement running through the Variety Zellweger piece - stars' faces are somehow ours as much as theirs - though the situation is markedly different for actresses thanks to Hollywood's deep-rooted fear of any un-tight female face over 40. It can't be a coincidence that the last film Zellweger made pre-"new look" was shot in 2008, when the actress was - yes - 39 years old. And after that? Seven years of nothing, then Bridget Jones 3.
Actresses' faces are watched differently, analysed differently, vilified differently. "Men look at women. Women watch themselves being looked at", is how John Berger put it in Ways of Seeing.
At a Star Wars convention earlier this year, Carrie Fisher admitted she broke down during the first screening of The Force Awakens because the HD picture had preserved her wrinkles in such pitiless detail. That might account for the digital retouching of skin on some of her close-ups - though naturally, the bark-like complexion of her male co-star, Harrison Ford, was left unbrushed.
The double standard was turned back on itself in Sunset Boulevard, Billy Wilder's black-comic noir from 1950, about a faded silent movie idol, Norma Desmond, plotting her impossible comeback.
"We didn't need dialogue," she snaps in one famous line. "We had faces!"
Norma was played by Gloria Swanson, a silent icon whose film career faltered when she reached the age of 39, and whom Wilder tempted out of retirement. When Norma mistakenly believes she's on the brink of second stardom, she submits to a grotesque beauty regime: electric shocks, steam treatments, pummellings, leather straps and masks.
The montage ends with a shot of Norma's own eye, monstrous under a magnifying glass, presumably being inspected for crow's feet - and for a split second, we too feel the profound unease of scrutiny in close-up.
Zellweger can doubtless sympathise. Even during her 20s, her face was routinely described in unflattering terms. But in films like Cold Mountain, Jerry Maguire and the first two Bridget Jones outings, it was essential.
Her face took the askance look at life her characters embodied - a smile tempered with a wince, or a stifled grin that suggested a plan was quietly percolating behind those crinkled eyes.
The best face in cinema this century belongs to Nicole Kidman. Specifically, it's Kidman's face in Birth, a film by Jonathan Glazer about a woman who believes her dead husband has been reincarnated as a 10-year-old boy.
About half an hour in, Kidman's character visits the opera: the camera finds her in the crowd, closes in, and watches her for two minutes in close-up.
As in Queen Christina, the background dissolves the nearer we get. Time passes. Music plays. She thinks, but doesn't speak. And you lose yourself in her.
The critic David Thomson wrote a much-derided but self-abasingly honest book about a life spent watching Kidman's face and body. But when it comes to that close-up in Birth - arguably the summit of his subject's entire career - he is lost for words.
"This is one of the great shots in surrealist cinema," he writes, continuing: "There is no kind of commentary possible to that scene: one cannot and should not [try to track] the way that her mind changes."
He's right. Whatever it is that happens in that scene, language can't capture it. The only way to understand is to stare.