Arnold Schwarzenegger isn’t invincible any more. Terminator sequels apart, it’s been 20 years since he starred in a film which broke the US blockbuster box-office threshold of $100mn. His seemingly stable family life similarly collapsed in 2011, when his wife since 1986, Maria Shriver, separated from him on discovering that he had fathered a child with their housekeeper. His initial popularity as California’s Republican “Governator” had also nosedived by the time he left office in 2011. As action star, politician and family man, Arnie entered his sixties on the ropes.

His unlikely response has been to reinvent himself as a serious actor, a move confirmed with the release of Aftermath, in which he plays a grieving and eventually vengeful father. Even more than his low-key role in Maggie (2015), where he quietly cared for his daughter while awaiting her transformation into a murderous zombie, Schwarzenegger shows gentleness, helplessness and nuance in Aftermath. Aged 69, he has finally learned to act.

Such subtlety hadn’t been needed in Arnie’s 1980s pomp. The macho American vision of the first Hollywood star-turned-politician, Ronald Reagan, had chimed with the blunt-force action films with which Arnie ruled the decade. The bulging pectorals on display as he wrestled Predators, leapt from a moving plane (Commando) and single-handedly shot a casino’s-worth of mobsters (Raw Deal) made him appear armour-plated. Even Clint Eastwood suddenly looked old, and lost his box-office crown to Arnie and his similarly pumped-up pal, Sly Stallone.

The actor who plays a grieving and eventually vengeful father in 'Aftermath', shows gentleness, helplessness and nuance in his latest role

Schwarzenegger had set himself ambitious goals ever since arriving in the USA from Austria in 1968, writing them on index-cards like a muscle-bound Gatsby. After being crowned as body-building’s youngest ever Mr. Universe, and ignoring his thick accent to triumph as not only an action but a comedy star (in Twins), politics seemed the inevitable next step in an irresistible rise. There was talk of changing the law, so this small-town Austrian could follow Reagan as President.

Schwarzenegger proved a less partisan California Governor than Reagan, but also provided an unwitting template for Donald Trump’s rise. There were “a heck of a lot of parallels” former Schwarzenegger communications director Sean Walsh told the Sacramento Bee’s David Sniders. “A lot of [Schwarzenegger’s appeal] had to do with one-liners, and…voter frustration”. Walter von Huene, his acting coach since 1996, helped hone political speeches to fit his star persona, meaning Democrats were derided as “girlie men”. Though they’d admired each other in the 1980s, ex-Governor Arnie then became a barbed critic of President Trump. But the Donald won that battle last month, when the actor quit as his replacement on The Celebrity Apprentice, after a single season.

A full decade away from leading roles meanwhile left Schwarzenegger’s Hollywood power exposed. There had been signs his lustre was fading even before he became Governor. He had co-starred in the disastrous bomb Batman & Robin (1997), as the thuddingly unfunny Mr. Freeze. End of Days (1999) then saw out the Millennium with Arnie in a punch-up with Satan himself. His character Jericho Cane was supposed to be an alcoholic ex-cop, but the role was beyond him. You half-expected his co-star, the method-acting great Rod Steiger, to bawl him out mid-film. As would become a pattern, ominously mediocre US box-office had to be compensated for abroad. Playing a clone in The Sixth Day (2000) and a clone of his Commando role in Collateral Damage (2001) didn’t help. Only Terminator 3: Rise of the Machines (2003) let him enter the Governor’s office on a high.

The former Governor of California starred in 'Commando' in 1986 during the peak of his action star days

When Schwarzenegger finally made his comeback with The Last Stand (2013), he soon found how fatally long he’d been away from multiplexes. Korean director Kim Jee-woon brought super-charged style to a High Noon-like plot, in which Schwarzenegger’s sheriff defends his town from a drugs gang. The part was “specifically designed for a 63-year-old broken-down guy”, an executive from the producing studio Lionsgate told Deadline; Jee-woon saw a new “vulnerability” in his star, who now suggested some of the later John Wayne’s rough dignity. Miserable US box-office of $12mn showed how few now cared.

A jail-break team-up with Stallone, Escape Plan (2013), showed a lighter touch from Arnie in its comic moments, but again needed Asian and European fans to turn a profit. Solo again as a corrupt cop in Sabotage (2014), embarrassing US box-office of $10mn confirmed Schwarzenegger’s action stardom was done. Even Terminator Genisys (2015), though a massive hit, was widely ridiculed. Playing an android who first crash-landed from the future looking like the Arnie of 1984 offered limited prospects for the weathered, sixtysomething ex-Governor.

Maggie wisely explored a new, more sombrely mature direction. Now Aftermath proves Schwarzenegger is serious about being serious. He plays Roman, a Ukrainian-American construction worker whose wife and pregnant daughter die in a plane crash. Blaming the air-traffic controller, he is determined to seek revenge. But Roman is no assault weapon-wielding tough guy. We see him early on in the shower, the then 68-year-old Arnie looking far from tautly super-human. He has a big, old working man’s lumbering gait and, discovering his family’s death, he’s polite, and helplessly angry.

The actor replaced Donald Trump on 'The Celebrity Apprentice' but quit after a single season

Why take such a role now – and why hadn’t he before, Aftermath’s British director, Elliott Lester, wondered when they met. “I was so busy being the biggest action star in the world, and then Governor, that it wasn’t in me,” Schwarzenegger confessed.

“In terms of raw skill as an actor, maybe when he was doing those action roles and being paid $30mn, the war wounds weren’t there yet,” Lester tells me. “I think you have to have a bit of damage to be able to do it. Although he would probably say that he has no damage at all. I saw in him a sensitivity. With all the success that he’s had, he’s been beaten up a bit along the way.” Lester knew he was leading his star into new terrain. “There’s a lot of baggage that comes with Arnold. I can’t ignore what he’s done before. But I was working with Arnold Schwarzenegger the actor from a small town in Austria. I was not working with Arnold Schwarzenegger from Bel Air. That’s not who I met. I’ve worked with people like Mark Rylance, and I found Arnold to be as professional and studied as any of those guys.”

Schwarzenegger’s stubbornly intact accent here suggests a man who’s still an American outsider. “100 per cent,” Lester says emphatically. “And he’s been that guy that Roman is. Arnold used to be a bricklayer.” These ordinary roots resurfaced on-set, and in his performance. “I found Arnold incredibly straightforward and simple in his tastes. We were shooting in Columbus, Ohio, and he wanted to go and eat German sausages and potatoes. And one day, we were shooting in a poorer part of town, and he went shopping for bargains in a store like Primark. That’s who he is.” When filming was over, though, Lester saw Schwarzenegger campaign for John Kasich in the Presidential primaries. “There were private jets, 21 security guards, and a convoy. He traversed two worlds, but was the same in both.”

Occasional acting coach Walter von Huene had perhaps renewed importance on-set. “Walter has been in his life for 20 years,” Lester explains of his little-reported role. “They go through all the emotional beats, and they read the script six, seven times a day, over and over. Walter also speaks German, and he’ll occasionally bark a word, but always check with me first. Back in the trailer, they sit and play chess.”

Lester equates the work ethic he saw in Arnie the actor to his body-builder’s devotion to “reps” (repeated exercises). “He did an amazing turn in Maggie,” Lester reflects. “It would be great, as he gets older, to have a film like Unforgiven or Gran Torino. I know he’s just done a comedy [Why We’re Killing Gunther].” It’s very late for such a second act, though. The model for Hollywood longevity, Clint Eastwood, built in such range-expanding roles from the start.

“It depends on how people react to these films,” Lester concludes, of Schwarzenegger’s commitment to leaving action behind. “I don’t know if his fans from The Terminator will necessarily appreciate Aftermath, which is quiet and intimate. But Arnold’s very self-aware. And I think that he knows that it’s very hard physically to do what he did in The Terminator when you’re 70.”