t's an hour before curtain-up and the queue outside the theatre is full of green wigs and blue faces, all amid a sea of black and white stripes. It is Halloween night and these theatregoers of all ages, snaking around a corner of the West End, are in themed dress for the musical Beetlejuice, adapted from the 80s film.

In other theatres' queues you might spot a king from Hamilton, the queens of Six and the wizards of Harry Potter and the Cursed Child, all bestowing new meaning to the idea of dressing up for the theatre.

There are overlapping reasons for this growing fashion for themed costumes. A new generation prefers active theatregoing experiences – there is also a boom in interactive and immersive theatre – and it is an extension of the fandoms facilitated by the growth of online forums and social media. Many fans find each other online first and then meet in theatres or at cosplay conventions. A major driver seems to be a desire for community and connection, ever since the isolation of the Covid era. Some shows combine the joy of dressing up with a sense of catharsis in exploring mental health issues together.

It's not just for the kids, either. As a fiftysomething critic, I started dressing in theme after a friend, the author Kamila Shamsie, suggested we don an outre ensemble for Burlesque just for the sheer campy fun of it. We then went to The Oresteia in ancient Greek costume (I wore a bat-winged charity shop cloak, Kamila a toga dress and foot pendant). I have since gone as a gothic governess for The Turn of the Screw and worn a black lace leotard and headband with ears for the musical Cats. My rules are simple: I only spend up to £10 and the point is to get in the mood and honour the spirit of the show without veering into fandom.

Some theatregoers go to extraordinary lengths to replicate the looks of their favourite characters, spending months stitching garments and creating props, and some musicals are better at facilitating this than others, thinks David Fynn, the star of Beetlejuice. "You can let your freak flag fly," he says. "It gives everyone permission to walk down the street and not be judged. There's a certain type of show that makes people feel comfortable to dress up. Rocky Horror was the first … Maybe people feel like outsiders together and the show makes them feel like they belong, so they go all in." From a performer's point of view, he adds, it is an absolute delight. "I can be having a tough day but to then see, as the lights go up, so many people who have gone to all that effort, you think, 'This is what it's all about.'"

Social media has played a key part in building the sense of togetherness, he thinks. "It's really supported those who may be too shy to turn up to Beetlejuice in cosplay on their own." I meet a trio of elaborately dressed ghouls in the queue who say they became friends online and that this is the first time they have met in real life. "We saw each other on socials and connected through TikTok," says Taylor, 32, who wears a floor-length Beetlejuice dress with fairy lights in netherworld colours embedded in it. "The show has the idea that a family is found," says Taylor. "As someone who doesn't speak to family, this really appeals to me."

Dorian, 25, looks magnificently macabre. They created their trouser suit with signature stripes using tape, and added decorative moss and mould, made from beads. Peeping out from behind is a forked green tail, and a last minute addition to the look was a ukulele made from cardboard and pins last night. "I love being able to dress up as this weird little ghost guy," says Dorian, who calls the musical "so queer and campy". Kirby, 29, dressed in a red spiderweb cloak, bat choker and homemade wig, says: "It's a home for all the weird kids and it allows me to express myself."

Musical Con has given people permission to go into auditoriums in character. The annual London theatre convention was founded by performer Shanay Holmes and partner Chris Steward in 2022, after a discovery Holmes made about the therapeutic effects of cosplay for her autistic younger brother. "He is a superfan of Star Wars and I attended a fan convention with him. It was the first time he had been, and he was in cosplay as Lando Calrissian. I saw some strangers approaching him, which would be a potential trigger for him, but because he was in cosplay he became completely empowered, and took on board the character of Lando… So we wanted to create an inclusive space where people are not judged for being superfans of a specific musical." Steward adds: "They come to Musical Con, and then 20 of them will go and see Phantom of the Opera or Beetlejuice in costume."

Could this surging interest create a new generation of future designers? "Some of the cosplay certainly looks like someone has snuck into a theatre dressing-room and taken a costume," says Holmes. "The intricacy and professionalism is incredible. Someone this year had made their own Christine Daaé shoes [from Phantom] – she had enrolled in a course to learn shoemaking! There's a massive shortage of technical skills, and it's these kinds of thing that encourage a foot into the industry."

A group of Hadestown fans relate how they dressed as its characters for Musical Con. Lorina, 25, recreated Persephone's dress over many months: "I tried to make a replica and went kind of insane in the process because there's lots of different versions out there. It took a lot of my summer and then some more." Eleanor, 19, speaks of "closet cosplay", which is about repurposing things already at home, for outfits for Operation Mincemeat and Beetlejuice as well as Hadestown. Jasmine, 21, dressed as Orpheus by wearing a white shirt originally bought for work, along with trousers from sixth-form days. "They were a bit too small in the leg but it worked well for Orpheus because the whole point is, as they sing, he's a poor boy. I borrowed the suspenders off my dad and the neckerchief was from the collection that belongs to my dog."

Some might argue that the booming trade in show merchandise is a cynical way to cash in, but fans speak of their emotional connection to this "merch": collecting, sharing or gifting it to fellow cosplayers. The musical Death Note, at the Barbican in London this summer, was based on Japanese manga – the genre that is believed to have spawned modern-day cosplay. Its director, Stephen Whitson, says that there were more than 200 people at the stage door, many dressed as characters and others wearing the show's merchandise, which gives "that sense of ownership and of joining a club". As associate director of Hamilton, he also remembers a teenage theatregoer who turned up and "sang the entirety of Hamilton dressed as Lafayette". Ultimately, the reason fans dress up is also why actors assume a costume on stage, Whitson thinks. "It allows them to locate themselves in this world and to become someone different – to walk in someone else's shoes."

Izzy, 21, is an actor who was drawn to the musical Heathers, which tackles subjects including high school bullying, depression, eating disorders and suicide. She was 13 when she first saw it and instantly connected with the character of Heather McNamara. "In theory, she is popular and has all of these things, but inside she's struggling a bit, and she's never truly happy." That resonated, as Izzy "was doing well at school, but I found it really hard to make friends and I was quite badly bullied. I'd never seen a show where I'd gone, 'Oh, this is exactly what it's like.' It takes you out of your reality, but you can still feel yourself within the show."

She has since seen the show 116 times and created more than 20 Heathers costumes. (Astonishingly, she has also put together the Don't Tell Mama dress from Cabaret, another favourite musical, by messaging the show's designer Tom Scutt, who talked her through his creation of the dress.) Heathers, she says, "has changed my life in ways that I will never truly be able to grasp. It's helped me overcome so many things and realise so many things about myself. And it's shaped the performer that I want to become, by watching that show and seeing so many evolutions of the same character."