He's in a band, has starred in an Oasis video, has been known to like a shandy, and is often to be found in skinny jeans and a biker jacket.
But now here he is, one of the stars of The Amazing Spider-Man, a mega-budget (reportedly $230 million), major studio, superhero movie. This is big business moviemaking, the kind of venture that expects its stars to sit up straight in interviews and mind their Ps and Qs.
Happily, you can take the rocker out of the band, but you'll never take the mischief out of Ifans. Whether he is riffing about the joy of being naked on screen (no more Notting Hill underpants), or delivering a four letter response to those who feel Shakespeare should only be spoken in a posh English accent, the 44-year-old Welshman still has a dragon-like fire in his belly.
In Marc Webb's picture, Ifans plays Dr Curt Connors, a scientist whose quest for the secrets of regeneration leads him to become The Lizard, Spider-Man's enemy. Ifans was seven and growing up in Ruthin, North Wales, when he saw his first Spider-Man comic. "On the back page there was a cut-out mask. I coloured it in, cut it out, tied it on." He laughs at how lo-tech it all was. "That's how old I am."
The son of teachers, Ifans trained at the Guildhall School of Music and Drama in London, also home at one time to Ewan McGregor and Dame Eileen Atkins. Films arrived late for him, with his first, the Swansea-set comedy drama Twin Town, coming along when he was 29.
"Movies were something Americans did. It was never on the horizon really. I always assumed when I started and went to drama school I'd be in the National Theatre, that would probably be the peak."
That would be the National Theatre in London. The National Theatre Wales was only set up in 2008; Scotland got its national theatre two years earlier. "Like everything else," says Ifans of the NT Wales, "we've had to fight hard for that. I'm sure you can empathise with that."
Ifans might have thought his future lay in theatre more than film, but then Notting Hill was released. Cue the character of Spike, Hugh Grant's slovenly house mate, cue that shot of Spike in his underpants on the doorstep, and cue the start of Ifans's tabloid years, when his life off screen, including a relationship with Sienna Miller, became the story. I wonder, 13 years on, how he views Richard Curtis's romantic comedy.
"I'll never, ever, regret taking that role as I thought it was a great role and I think it's a great film." The only hitch was he was offered lots of similar roles afterwards. "When you are young like that, and have an opportunity like that, I jumped at everything." Which perhaps goes some way to explaining Rancid Aluminium and Kevin & Perry Go Large.
He took a path away from Spike, via Harry Potter, Greenberg, Anonymous, theatre and much else, that led eventually to Spider-Man. "I guess a nine-foot reptile is as far away from that as I could possibly have got." And, as a nine-foot animated lizard, he didn't have to wear underpants, so no link with Spike there. "Finally, I'm fully naked," he laughs.
Ifans left behind any notion he was not a serious dramatic talent when he played Peter Cook in the 2004 television movie Not Only But Always. Though he won a Bafta for his brilliant take on the British satirist, he had turned down the role several times, fearful of what the late Cook's friends might say.
"This guy was a genius and I thought, hang on a minute, if you screw that up, and most of his compatriots are still alive, I'd be lynched by a lot of very clever English people, you know? But I caved in the end."
There was a lot of written and audio material on Cook, but very little giving a hint of the man himself.
"As gregarious as he was, I discovered there was a deep sense of otherness and loneliness to him. Which of course made him a great observer. He was a complete outsider, even in the midst of everyone. On one hand, that empowered his satire and comedy, but on the other must have been very lonely. To be lonely in a crowd is the loneliest place in the world because it's not a loneliness you choose. It's not solitude, it is loneliness in all its coldness."
Judging by the way his eyes fill up, the role remains important to him both professionally and personally.
"I still to this day get quite moved talking about him, because I felt like in a really strange way that I got very close to someone I'd never met. When I finished it I felt this huge sense of loss. I did go into a kind of grieving."
In his upcoming film, Serena, with Bradley Cooper, Ifans plays an Appalachian bodyguard. He's also in HBO's adaptation of Jonathan Franzen's The Corrections, alongside Anthony Hopkins. Though his American accent, like his English one, is perfect, Ifans is proud of his Welsh heritage. Welsh is his first language. Did he ever worry that his accent would count against him?
"There was that initially. Richard Burton's generation had to lose their accent or they wouldn't have got a look in." Then the likes of Tom Courtney and Albert Finney came along and made accents acceptable.
"Even when I was in drama school there was a climate of, 'Oh no, no, you can't go into the RSC with that accent. You must lose it.' Well, I am of the school of f*** that, because I sound better in my accent." Hearing Shakespeare in a Welsh accent makes more sense, he says. Or Macbeth in a Scots accent, I say. "Yeah, come on."
Odd thing about the Welsh, though. They kept their language, but haven't been as supportive of independence as the Scots. "There's a geographical element to that. Our border is an open wound. You luckily have a few mountains between you and them.
"Every culture has to jettison something. We hung on to our language and our language became our judiciary, our royalty.
"We've got a lot to learn from each other. I do wish the Welsh nation would have the same vigour for independence that Scotland does. It's good to know you're there."
Ifans, once an early member of the Super Furry Animals, retains his rock links. His band, The Peth, have an album ready to be released.
He doesn't need to worry about the video, going by his performance in Oasis's The Importance Of Being Idle, when he displayed more swagger than Jagger and Liam Gallagher combined.
"There's no reason in the world why we have to specialise as creative people. I like cooking but that doesn't make me a restaurateur." Does he fancy a cooking programme then? "No," he laughs. "It would be one show – spaghetti bolognese."
Spag bog, Shakespeare, Spider-Man: what a rocking, rolling web Ifans weaves.
The Amazing Spider-Man is out now