The only well-known ‘background actors’ are the ones who made epic mistakes while the cameras were rolling: the Stormtrooper who hit his head on a door frame in A New Hope. The kid who covered his ears before the gunshot in North by Northwest. The guy who swept his broom inches above the ground in Quantum of Solace. And the brave fool who threw a beer can at John Malkovich’s head in Being John Malkovich.

Fictional extras aren’t much better: Ricky Gervais as a barely-likable failure in his show Extras. There’s a recurring French and Saunders sketch where two extras ruin every scene they’re in; and in the opening scene of The Party the character played by Peter Sellers character blows up an entire set.

An extra’s job is to blend into the background of the action and that makes it very easy to underestimate them. But that’s not what I saw on the set of Fantastic Beasts and Where to Find Them.

What I saw was a secret society of people who cannot live without film. They’re smitten with it, and they’re ultimately no different to anyone else who works in the biz: they work long, weird hours loitering in waiting rooms day after day, in full costume, for the chance to be a part of the magic.

Frankly I think they’re the sweet, unsung heroes of the movie business.

They’re such a fascinating bunch, too, if you get to talking. Especially the career extras: the people who’ve been at it for decades and love it like it’s their very first day. It’s enchanting.

Going undercover as an extra was basically like conducting my very own sneaky study in human nature. The ultimate on-set social experiment: put 100-odd people in 1920s costume, hair and make-up, confiscate all technology, provide biscuits, leave them in a giant tent for hours at a time and see what they do.

It’s like Survivor: The Movie Set Edition; alliances are made and broken, enemies made, romances begun, feuds started between similar-looking bald men vying for the part of Bald Man Number Four. People talk. They debate. They tell extravagant stories about that time Harrison Ford brushed their arm.

They splinter off into different tribes. There are the nappers. The readers. The knitters. The gossips. The cryptic crossword posse. The hours-long card game players. The spontaneous yoga teachers. The compulsive snackers. The very serious actors practicing accents they’ll never get to use in this film.

On Fantastic Beasts, there was a former military helicopter pilot crocheting the sleeve of a jumper in between takes playing a waitress serving Colin Farrell. There was a reformed biker who had a near-death experience six years ago and now only values family, health and movies. The cherub-faced illustrator sketching faces in a notebook, the American who read the whole of War and Peace on the last film he worked on, and the two most elegant women I’ve ever seen – both in full witch costume and at least 80 years old.

What I’m saying is this: yes, being an extra is a lot of standing, sitting, walking, pretend-talking and discreet-eating in the background of a shot. Yes, there’s a lot of killing time. And yes, it’s expensive agony if someone ruins a single take or gets fake snow on the back of their vintage suit.

But one of the greatest things I’ve learned during my crazy-beautiful time on the set of this film is this: movies don’t get made without the little-known loveliness of extras.