Scientists trying to capture the essence of sex appeal might like to experiment on Jared Leto, since his is contagious – after spending a day in his company, I’ve got women falling at my feet too. Sadly, it transpires that the 19-year-old Spanish woman who just plunged on to my shoes has in fact fainted. A devoted fan of Leto’s band, 30 Seconds to Mars, she has just watched their show at the As-toria in London, and the thrill has proved too much.

She had already seen four of their shows in a row after saving up every last euro to follow the band around Britain with other Spanish obsessives whom she met on the band’s internet fan sites.

The ten-strong Belgian contingent that I speak to have done likewise – and these are just two nations within “The Echelon”, as devotees of Leto’s punk-rock band are known.

“A giant dysfunctional family” is how Leto himself, tucked into a grotty dressing room with his band before the show, proudly describes them. He explains in some depth what an unusually strong relationship they have with their audience. “The word fan does come from fanatic,” points out a grinning Shannon Leto, Jared’s brother and the band’s drummer. But these fans are beyond fanatic. These fans are mad.

They will say that it’s because they love the pomp and circumstance of the shows so much, full of jumping and strutting and power singing, or that they identify with the message of the songs, but clearly, the secret lies with Leto. Better known as an actor, he rose to fame in the cult teenage drama series My So-Called Life,going on to act in films including Fight Club, Requiem for a Dream and Alexander.

His name is often to be found in the gossip columns of celebrity magazines, and his little-boy-lost eyes and all-American charm have had him romantically linked with Cameron Diaz, Scarlett Jo-hansson and Lindsay Lohan. When you meet him, it’s those cerulean orbs that strike you first, though the faultless manners come swiftly afterwards, as do those of his bandmates. It’s hard to imagine a British band allocating so much of their budget to charm school.

No wonder wellwishers used to hurl money at the teenage Leto busking on the streets of New York after he escaped from a not altogether happy childhood, in which he and Shannon moved all over America. “I've had so much edge in my life, I don't need to contrive any,” he once said. He studied art, then film-mak-ing, before the bigtime of My So-Called Life.

Leto recently returned to the streets of New York to play Mark Chapman in a bio-pic that seeks to humanise the man who killed John Lennon; he gained and has now lost lost five stone, 32kg, for the role. He says he wanted to “investigate the humanity behind the monster”.

Filming took only six weeks, though, and then he was back to the two-year-long touring duties of the band, who have now sold more than a million copies of their second album, A Beautiful Lie, in America. They were signed in 1998, and initially refused to let Leto’s name be mentioned in their publicity, which meant that they had to take the long and sweaty route to success. Even now, it’s clear that he can’t VIP his way out of the hard work. On the day I spend with him there’s a lengthy soundcheck, which he describes as constant problem-shooting, then interviews, a photoshoot, at which a make-up artist touch-es up his nose where it got broken in a crowd stampede at a gig in El Paso, and a private audience with a competition-winning fan, who gives him a red rose whose thorns she says she has pulled off specially (he later passes it on to me, “regifting the gift”).

Then there’s a meet-and-greet with record-label types anxious for a dusting of Leto sparkle, the ludicrously energetic show, and then two whole hours of signing autographs for fans (nobody is ever turned away).

By midnight, he is pressing the flesh at the aftershow drinks (though he steers cleer of booze these days), and then, when it starts to get light again, there is the small business of a flight to Austral-ia to do it all again. He also designs the band’s artwork and directs their videos (under a pseudonym), choosing to film them in easily accessible locations such as the Forbidden City in Beijing.

Renaissance man he might be, but where does the drive come from? Is it all just another of his acting roles? When it comes to the music, he says not. “On the contrary, I think that when you go on stage, you become more of who you really are. I think it’s a side of yourself at the very least.”

The band is very stagey though, he agrees. “Yes, we have a certain element of gre-gariousness, of theatricality. It’s definitely more art rock than it is punk rock. And I think a return to some size and scope in rock’n’roll is something to celebrate. A little bombast; a little larger than life – we like to explore ideas and not to be constrained by what we think is cool or not cool.”

Coolness is an interesting point, because the odd thing about Leto’s musical project is that it is not perhaps as cool as Leto himself. The music has now found genre fellowship with emo bands such as My Chemical Romance, but as Leto points out, 30 Seconds to Mars have been going far longer and have always been “the odd men out”.

Their fans are not particularly glamorous; they seem to find solace in friendships largely struck up through the internet that they cannot find elsewhere. Why would a man who can date Scarlett Johans-son want to connect with these people so much?

“Because it’s nice to listen to people that have made a deep connection with other people through a shared experience. You can’t force that to happen, you can’t create it. But when you try to give a reason why it’s happened you end up sounding . . . is ‘naff’ what you say in England?”

The cynic would suggest that it’s because they offer unconditional devotion, and there is indeed something a little messianic about Leto.

As my new friend from Barce-lona told me, “This isn’t a group, it’s a cult!” And then she fell over.

The single The Kill by 30 Seconds to Mars is released by Virgin on Monday

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