Twenty-five years ago, Mark Sinclair began work as a bouncer and was asked to change his name. Something tough. So: Vin for Vincent, his stepfather's surname. And Diesel because that's what everyone said he must run off. Even before he got a job that required routine speeding, meting out summary justice at 100mph, pumping easeful machismo in a succession of vests, people detected petrol in his veins.

This Friday teatime, Diesel is in need of a transfusion. He staggers into the hotel suite clutching his crotch in pain – horseplay with his entourage, he explains – then lies on his side on the sofa. And stays that way, like a groggy mole, mildly smiling, eyes squeezed shut or fluttering open. His voice, the bass purr of a 1920s Broadway impresario, is today so deep, so quiet, some of his words stay between him and the whales.

Why so tired? A few days ago he wrapped work on Fast & Furious 7, now he's doing international press for new movie Guardians of the Galaxy, then he'll start shooting his next, about an undead witch hunter.

"You do what you love and it's not work." A friendly yawn. "That's my philosophy."

I'd read another: "Shut your mouth, watch your back and keep working 'til your ass falls off."

"That too."

So how's his ass?

"It is sagging a bit," he grins, quasi-cockney. He swivels round on the sofa to better inspect. "Is my bum right?" Last night, Diesel was springing about 10ft tall, showing up to the London premiere of Guardians in high-tech stilts and an "I am Groot" T-shirt. The stilts were to show how he did the motion-capture for his character, a genetically-modified tree who joins a gang of space-age scrappers to save the world. "I am Groot" is his only line of dialogue.

A monosyllabic tree isn't everyone's idea of a dream role. Yet it offered an escape from his own body. "It appealed to the thespian in me. You're talking about a very challenging character to pull off. How could you really create a whole character with three words? To really bring the emotion and chart the arc."

The experience was, he says, "peaceful. I guess I never really realised how much I did always love trees." As a child, he used to "fantasise about these great old weeping willows"; since seeing the film his son, Vincent, three, refers to trees as his "brothers and sisters". Diesel beams beatifically. "There's so much wisdom in children when we pay attention." He means it – Vincent pointing to Groot in a comic book was the clincher when considering the part.

Guardians of the Galaxy, for all its intergalactic bells and whistles, isn't so different from many of his movies: a motley crew of soft-centred crims bond to become closer than family. It's also a melting pot, so multicultural it doesn't even matter if you're animal, vegetable or mineral (Groot's best buddy is a raccoon voiced by Bradley Cooper). Would it be true to say it's set in a post-racial world?

"So true," he says. "On a deep level. We're dealing with species at this point. We've gone beyond."

His eyes open wider. One day, he hopes, discrimination will seem like a blip. "Race is a new word, relatively. It's like 100 years old. It's something that kinda just fed the Californian eugenics movement." Diesel – and twin brother, Paul – were born in Greenwich Village in 1967 to Delora, a white astrologer, and a father who Diesel knows little about other than that his parents' relationship would have been illegal in some US states under anti-miscegenation laws. He was raised by Delora and his African-American stepfather, a theatre manager called Irving; he invokes both frequently and reverently.

He broke into a theatre with friends aged seven and was offered a part in a play, began breakdancing and weight training in his teens, studied screenwriting at college, then spent nine years auditioning by day and working as a bouncer by night. His 20s were professionally unsuccessful; his body and its pulling power, he's said, were one of his few sources of self-confidence.

The sticking point for casting directors seemed to be his indeterminate ethnicity, forever too light or too dark for the part. So he decided to use it; took a second job selling lightbulbs to fund an autobiographical short, Multi-Facial. It played at Cannes in 1994 and caught the attention of Steven Spielberg, who wrote him a role in Saving Private Ryan. It was in drama, then, that his career began: a gangster in Sundance-selected Strays, the voice in The Iron Giant, a ruthless stockbroker in Boiler Room. But it wasn't long – Pitch Black and xXx were both in 2000, The Fast and the Furious the next year – before his aptitude as mysterious, ripped anti-heroes with a strong sense of conscience and a nice drawling delivery meant his move into Brando territory had to be put on hold.

Diesel has said he finds having an identity fixed on paper reassuring. What's curious is that this identity has remained so static – and so different from how he seems in the flesh. For Diesel is neither stern nor formidable, but fun, vulnerable, even a little camp.

Groot, the sweet, dumb, gummy stump is, he thinks, the closest match so far between his personality and a part. The innocence especially. "To maintain your innocence is a feat. So much magic comes from it. Ironically, people probably see more of Groot when I'm posting than they would in any of the characters I play."

This posting is on Facebook, and Diesel's page – updated daily with photos, shout-outs, mantras – is largely a cynicism-free zone. He's currently the fifth most popular celebrity (and most popular actor) across the site, with 82 million likes.

Last year, he suggested that Facebook undervalues celebrities as traffic drivers, and that many studios – and stars – failed to appreciate the power of such direct interaction. "Imagine if you were able to Facebook Elvis, and talk to him, and hear from him without the Hollywood of it all. That was the Fast & Furious experience."

And the virtual accessibility of that franchise's leads was indeed a factor in its extraordinary success. When Paul Walker died in a car crash last year, people mourned in a manner proportionate to any other loved one who appeared in their Facebook feed. And it was on social media that people, including Diesel himself, came to grieve.

The comfort offered by his page is reciprocal, he says. "The wisdom on the page is profound and therapeutic." Pressurising, too, perhaps. He speaks of movie promises made online which he'll feel "always backed up, always in debt" until he fulfils – in particular a trilogy of films about the legendary military commander, Hannibal.

Why does he feel so beholden?

"Maybe because I believe that my movies affect people? That adds a significance and an importance to my work."

Does he think he believes that more than his peers?

"Probably."

Why?

"Why do you think?"

Maybe you care?

"Maybe you care. That's it. What a unique thing to say in 2014." He sits up, grips the sides of the sofa with piston limbs. "A couple of years ago I quoted my mother on a post, something like: 'People caring for people.' And my fans ran with it like it was a profound thought. And it's sad, really, that something like that would be quotable. That it would need to be said."

Guardians of the Galaxy in on release in the UK and is out in the US on 1 August.

• Peter Bradshaw on Guardians of the Galaxy: 'Smart and funny'