Daniel Portman is soaked in sweat and locked in battle. There is an epic swell of music as he lunges at his opponent. But we’re not in Westeros, where Portman became a Game of Thrones fan favourite as steadfast squire Podrick Payne. He is centre-stage at the Edinburgh fringe, in a red unitard, arm-wrestling an audience member. Portman goes over the top, the crowd cheers and he laps up the applause.

This is Square Go, a late-night rumble of a show in which he plays 13-year-old Glaswegian daydreamer Max, hiding in the school toilets with his mate Stevie, awaiting a fight with a bully. When we meet in an Edinburgh pub, I ask Portman if he always wins the arm wrestles. “Generally, yeah,” he says instantly, with easy confidence, in a voice as deep as his eyes are brown. Occasionally, some “big macho bloke” gets carried away, in which case Portman will remind him: “You’re fighting a 13-year-old boy who’s doing this in his fantasy world. So I’m gonna cheat to win.” He smiles. “Some people take it really seriously.”

This is what the show is about: how boys hide behind bravado right through adulthood. But there’s no lecturing or hand-wringing. At the start, Portman strides in like a wrestling pro, getting the audience to feel the steel and chant his character’s name. Max is psyching himself up for his “square go” – Glaswegian slang for a fight – but soon crashes back to reality. “You’re gonnae get pure tabernacled,” Stevie reminds him.

‘Pure tabernacled’ … Gavin Jon Wright and Portman in Square Go, directed by Finn Den Hertog. Photograph: Mihaela Bodlovic

A high-octane hour, Square Go is performed in the round. “It’s exhausting,” says Portman. “The energy is like a vortex.” He’s superb in the play – as is Gavin Jon Wright as Stevie. It’s clearly a hoot to perform. Portman and Wright, who were in Black Watch together a few years back, have nailed the comic timing of the “your maw” gags and putdowns that come thick and fast in Kieran Hurley and Gary McNair’s script. In this very physical role, Portman proves a nimble comedian, trembling as he bobs and weaves. He is slammed to the floor repeatedly by Wright and mercilessly whacked with a blow-up wrestling doll.

Everything is OTT, including a thumb war staged like an arena-sized showdown. “When you’re that age, your universe is school,” says Portman. “It is funny that it’s so grand and operatic in scale but it’s nuclear to these guys. This is the end of the world. The way these characters in Max’s mind are portrayed is slightly ridiculous – the luchador mask, inflatables, a guy with a big foam finger – but at that age you feel everything more.”

Portman grew up in Glasgow and had “a leftwing upbringing, open-minded and fair”. His dad is the actor Ron Donachie, his mum a complementary therapist. Could he relate to Max? “I never felt any pressure at home but outside the house I did feel pressure to be tough. The only acceptable emotion outside the house was anger, if there was anything. Sadness wasn’t allowed.” He says he was arrogant: “I was naturally quite big and athletic and I certainly imposed myself on people.” It was through insecurity, he acknowledges, but then borrows a line from the play: “Every dick’s got a reason for being a dick – it doesn’t change the fact I was a dick.”

The teachers who told me not to do acting asked me to talk to pupils about how much the school gave me. No chance …

What was Portman up to at Max’s age? “Kicking a football around, having a pretty good time of it.” Did he have his own square go? “I got into quite a few scrapes when I was young. They always made me very sad.” In hindsight, he says, he’s sort of glad. “Some men, if they haven’t had that experience, they maybe carry it into adulthood. They still feel like they have something to prove. I don’t have that.” By the end of Square Go, the venue becomes stifling and reminds Portman of school fights: “If something was going to kick off it got hot. You could smell it.”

The end of Portman’s school experience was soured by “some very nasty rumours spread about me by a couple of guys who were very close to my heart”. He says he ended up with almost no friends. Part of the problem was that he’d embarked on an acting career. “It makes people uncomfortable. They go, ‘Do something real.’” Apparently, this extended to the staff, with the exception of his drama teacher. “The same teachers who told me not to do it have asked me to go back and talk to pupils about how much the school gave me. Go fuck yourself. There’s no chance. If I do come in, I’ll say, ‘Don’t listen to these people.’”

‘When you’re that age your universe is school’ … Daniel Portman in Square Go. Photograph: Mihaela Bodlovic

This is his first summer performing at the fringe, and Portman feels as if he’s cheating because his show is selling out and he doesn’t have to flyer it. He remembers travelling from Glasgow when he was younger and watching free shows back-to-back all day. He’d grown up seeing his dad on telly – Casualty one night, The Bill the next – and the big screen, notably as the master-at-arms in James Cameron’s Titanic. Donachie even went on to have a small role in Thrones, as Ser Rodrik Cassel.

Portman made his acting debut with a youth theatre group – playing Moonface, a gangster disguised as a priest – in the Cole Porter musical Anything Goes. Looking at him today, dressed in black with an open face that seems both solemn and saintly, yet has the widest smile, the casting still makes sense.

That production’s boozy last-night celebrations had disastrous results. “I gave my then girlfriend my jacket. By the time we got to the after party I was fucked. I woke up the next day and had pneumonia. I was out of action for a month.” On his first day back to school, there happened to be a casting session for a Scottish horror film Outcast, starring James Nesbitt. Portman auditioned on his 17th birthday and got a walk-on part.

Loyal servant … Portman as Podrick, with Gwendoline Christie as Brienne of Tarth. Photograph: Sky Atlantic

What happened next? A bit of theatre at the Òran Mór in Glasgow, the BBC Scotland soap River City and “then Game of Thrones came up”. The biggest show on the planet appears in conversation as casually as it pops up on Portman’s sparse IMDb page. Was he intimidated, arriving on set? Nervous, yes, he answers. “But not overwhelmed.”

Was he daunted by the now famous orgy scene, where Podrick gives a group of prostitutes such pleasure that they refund his payment? “When you’re 20 and on a big TV show and the writers tell you your character is about to become the greatest lover in the world” – he pauses – “you tell me how you’d feel!” The scene led to some “interesting journeys” he laughs. The reports earlier this year about him being groped by fans at conventions were blown out of proportion he says, but adds: “It has happened a few times, with men and women doing it. It has given me a bit of an insight into what women have to deal with sometimes and it’s very, very unpleasant.”

Portman first auditioned for two characters who were bumped off in season two. The producers invited him to audition instead for Podrick, who survived to season eight. After the series wrapped, he was told Podrick was meant to be in just two or three episodes but they “liked having him around”.

Podrick is a loyal servant of Brienne of Tarth, the resolute warrior played by Gwendoline Christie. She tells me Portman has “a wonderful capacity for enjoying the absurd”, which was useful during “long days swathed in mud or submerged in water wearing leather and metal clothes”. Does she think Portman has anything in common with Podrick? “A mellifluous singing voice,” she responds, “and impossibly jutting cheekbones.”

‘If something was going to kick off it got hot’ … Daniel Portman in Edinburgh. Photograph: Murdo MacLeod/The Guardian

Portman auditioned in his own voice and in received pronunciation, and the Thrones producers chose RP for Podrick. “There weren’t any Scottish accents in the show,” he says. “I don’t really know why not. Maybe they were worried people in the States wouldn’t understand.” The majority of his encounters with fans are friendly but “people do take the piss sometimes and think they can push me around. They expect to get Podrick Payne. They expect this shy, timid, little English boy. That’s not what they get.”

Portman’s next stage role is in The Last King of Scotland, an adaptation of Giles Foden’s novel, in which he will play the medic who becomes personal physician to Idi Amin. It opens in Sheffield in September. For now he is clearly over the moon to be in Edinburgh. His Thrones pal Joe Dempsie and he were having a few drinks the other day and were handed flyers for Thrones! The Musical Parody. “They say they ‘fix the final season’,” he observes. He’s planning to pop along. “I just hope they don’t try and drag me on stage.” Meanwhile, he’s looking forward to his family seeing Square Go. He cracks a mischievous smile. “I’ll see if I can get my old man up for the arm wrestle.”

• Square Go is at the Roundabout, Summerhall, Edinburgh, until 25 August.