BBC News looks behind the scenes

On a freezing November day in 2023, we drive to a school on the outskirts of London. It looks unused and unremarkable – but then a sign that says “Truham High” gives the game away.

This abandoned-looking building is actually the setting of the Netflix sensation Heart stopperand on the day of our visit, filming for the third season is in full swing. The new episodes will be released on October 3rd.

Inside, the hustle and bustle you'd expect in a school hallway was replaced with the organized chaos of a movie set. The school's simple interior is covered in colorful murals painted in the style of Alice Oseman's graphic novels that inspired the show.

Mr. Ajayai's art classroom is immediately recognizable.

In the first two series it was a safe place for Joe Locke's character Charlie to turn to when he was having a hard time. During our visit he is joined by Kit Connor, who plays Nick – and the conversation they film looks difficult.

They sit in a corner and talk in hushed tones.

While the producers remain tight-lipped about the details of their filming, they tell us, “It's a pretty intense scene.”

They protect the number of people in the room with the actors during more emotional scenes. Dozens of silent crew members are crammed into a nearby room, watching intently on monitors as the actors do multiple takes.

The third season of the show deals with more serious themes than the first two; Charlie's eating disorder will be a central storyline and some characters will have sex for the first time.

“We've always said that the show is growing up with the characters, which is definitely the case this year,” Locke tells us during a brief break before his next scene.

“The show deals with some heavier themes like mental health and growing up, so there's a lot of 'teenhood'.”

The show has been praised for showing “queer joy” – highlighting the positive aspects of growing up as part of the LGBTQ+ community – but Locke says it's important to make sure it's still realistic.

“It's about authenticity and making the portrayal of a subject that is very intense and very close to people's hearts as true as possible,” he says.

Connor and Locke joked that it's hard to get used to kissing a boyfriend on camera, and since the actors are all so close to each other, they admit that some of the more passionate scenes can feel strange to film.

Connor tells us: “When you shoot for seven and a half hours, like we did for a slightly steamier scene, the question becomes: What is my job?”

During our visit, it was clear that the chemistry between the actors extends beyond their time on screen. During our interviews, they joke and gently poke fun at each other, which Connor says helps on the more difficult days on set.

“After filming, we all just crowd into someone’s apartment and spend time together. We have a great opportunity to do that without talking about work or anything like that,” he says.

“We all just go home, cook dinner and criticize each other’s cooking.

“(Joe Locke) actually made a good chili this time… last time it was a very, very bad chili. This time, Will, who plays Tao, had seconds… but he’ll eat everything.”

While the friendships may be real, some of the series' most famous locations are actually flat-packed and built as needed within the school.

During our visit, Nick's bedroom is being set up in the school's former sports hall.

There are no lights as there are no scenes planned here today, so we explore the area with our cell phone flashlights instead. Even in the semi-darkness, the attention to detail is clearly visible.

From books about bisexuality to Polaroid pictures of Nick and Charlie together, they've gone to great lengths to make the bedroom a true reflection of the character's journey over the course of the series.

“The fans really notice everything and have theories about even the smallest details… You should check out some of the TikToks they make,” a crew member tells us.

Just down the hall from Nick's flat-pack bedroom is the costume department, where designer Adam Dee says he likes to give mega-fans things to look at.

“If they have an open wardrobe scene, we will incorporate some iconic pieces from previous series so the audience can recognize them,” he says.

“With Elle, she has a sewing station in her room, so we managed to add some bits and pieces to her wardrobe made up of two other pieces that we sewed together or vintage scarves that we turned into tops.”

“Heartstopper” was something of a surprise hit for Netflix and the first season launched with very little advertising.

It became one of the ten most-watched English-language series within two days of its release and received an almost unprecedented 100 percent rating on Rotten Tomatoes – something many critics attribute to this diverse display on the screen.

In the third season, some of the show's older characters turn 16 and begin having sex.

Yasmin Finney's character Elle, who is transgender, is shown beginning a sexual relationship with her on-screen boyfriend Tao (William Gao).

The sex lives of transgender characters are often left out of television storylines.

Finney tells us: “It's nice to have a representation of a trans person who is able to have these kinds of relationships.

“It’s very sweet and I’m very blessed to be able to do this with Will.”

In the second season, Tobie Donovan's character Isaac discovers that he is asexual, meaning he experiences little to no sexual attraction. It's another topic that few television shows have covered.

Donovan tells us he felt like he wanted to “do a lot of research” to make sure he “does justice to this community.”

He added: “There's never been anything like this on TV so I really wanted to make sure I got it right.

“I feel like based on the response to season two, people were pretty happy that they had someone on screen telling a little bit of their story. It’s great for all of us.”

Just before we leave, we see the show's executive producer, Patrick Walters, on the set of Mr. Ajayai's classroom.

Walters had the idea to turn around Alice Osemans Adapting graphic novels into a TV series after he “fell in love” with the books.

He is particularly proud of the diversity of the show.

He says: “It’s amazing that it’s an important show for LGBT young people.

“I like the idea that young people really see themselves in the characters and that’s why it connects.”