The Devil Wears Prada 2 has a cameo list more stuffed than the fashion cupboard at the film’s fictional Runway magazine. It runs the gamut from eye-poppingly famous (Lady Gaga, Donatella Versace, Naomi Campbell) to if-you-know-you-know industry famous (Tina Brown, say, or a host of supermodels familiar to anyone on the Paris front row) to “huh, how did they get there?” (Late Show bandleader Jon Batiste, or Chicken Shop Date’s Amelia Dimoldenberg, already on her second cameo of the year after a super-quick turn in an episode of Industry). Missing, though, is the one cameo everyone hoped for, the white – or should that be cerulean? – whale herself: Anna Wintour, Vogue top dog and heavy inspiration in the film for Meryl Streep’s formidable sadist-in-chief, Miranda Priestly.

Wintour, though absent from the original Devil Wears Prada, always hovered over proceedings – it’s said that a number of designers steered clear of cameo appearances in the first film for fear of offending her – and Wintour herself, though present at its premiere, always studiously avoided discussing the film. But in recent months there seems to have been a sudden thawing – fond words from Wintour about the film on the New Yorker podcast, then a shock appearance alongside Streep on a Vogue cover – prompting speculation that the be-fringed one might deign to appear in the sequel.

It wasn’t to be: Wintour did film a scene, but according to director David Frankel it was left out of the film after she “jumped her cue”, and Frankel didn’t dare ask her to do a second take. That’s probably for the best. Wintour’s greatest superpower is her unknowability, her unapproachability, swanning around in elevated circles we no-marks can only gawp up at. To have her rocking up at Milan fashion week and lowering her large Chanel sunglasses to deliver a knowing look towards Miranda would chip away at her glassy aura, while at the same time turning the film’s long knowing wink of a homage to her into heavily signposted mugging and gurning.

The benefits of a well-placed cameo can be massive. For the star doing the cameoing, they are usually quick, and highly profitable; for the movie that the star is cameoing in, they can serve as a power move, suggesting an easy glamour, a rarefied space in which incredibly famous people casually swan about. The best films incorporate them seamlessly: the 65 film star cameos in Robert Altman’s Hollywood black comedy The Player are there to make the world feel as lived in as possible – with the added bonus of inserting someone as major as Burt Reynolds or Cher into the film. 30 Rock did something similar with its endless stream of A-listers whizzing in and out of the revolving doors of NBC HQ in a surrealist whirr, though some would argue that it was the sheer flurry of celebs that ultimately did for the show, distracting from its primary function as a very funny sitcom.

A bad cameo tips that feeling of easy glamour into something forced and even a little desperate. At their worst – when someone starry rocks up briefly in a film, the camera lingers on them a little longer than necessary, and someone shouts, “Hey, is that Kim Kardashian/David Beckham/the Dalai Lama?” – they completely halt the forward motion of a movie. Even worse is when said cameo-er hasn’t the slightest hint of acting ability: Ed Sheeran (pictured above) gormlessly gazing into the screen in his brief Game of Thrones appearance springs to mind. Imagine if Wintour had been that wooden? Her decades-long spell would have been instantly, irrevocably broken.

That said, what is, for my money, the funniest movie cameo of all time does all of the above: it’s unnecessary, completely disrupts the plot of the movie, and features a performance that is a little wooden. But despite all this, it still works. I’m talking about Marshall McLuhan, the celebrated media theorist, crashing into a fourth-wall-breaking scene in Annie Hall to tell the pseud in the cinema queue who has been chin-strokingly referencing him that “you know nothing of my work”. In fact, it’s a withering put down that feels more than a little Wintour-ish. Shame they got there first, eh, Anna?

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