If the baddie is the best part, then Douglas Henshall has certainly bagged the best of the best. He plays Satan in Stephen Adly Guirgis's The Last Days of Judas Iscariot at the Almeida Theatre and you don't get much badder than that. Hence the little scrap of ginger stubble he is hoping to turn into a thoroughly modern devilish goatee for opening night. “No horn and tails; I'm going for the debauched lounge singer look. Dean Martin, but with an edge...”

The 42-year-old Glaswegian's career is finally thriving. In the past decade Henshall has progressed through credible small films such as Orphans to ambitious, challenging TV roles such as the father of a child with attention-deficit hyperactivity disorder in The Kid in the Corner. Now, as Professor Nick Cutter in Primeval, he is currently ITV's very own Scottish sci-fi hero. The Beeb can keep David Tennant. Sipping water during a rehearsal break, Henshall compares the rollicking series to another show: “I think it's more The A-Team with dinosaurs.”

Henshall's welcome rise has been steady rather than meteoric. But then acting was hardly in his genes. His father worked for a magazine company and his late mother was a nurse. At school Henshall enjoyed performing on stage almost as much as he enjoyed football and at 18, after working with the Scottish Youth Theatre, he joined Mountview Drama School in North London. After he left Mountview the television roles grew, and it looked as if he was about to break big after an early appearance alongside an unknown Ewan McGregor in Dennis Potter's Lipstick on Your Collar in 1993.

It did not quite happen, but by the late 1990s he was making a name for himself in light romantic movies such as This Year's Love and hard-hitting TV dramas such as Psychos, set in a psychiatric hospital. Stage roles in The Life of Stuff at the Donmar and American Buffalo at the Young Vic made his theatrical CV shine. By the end of the millennium Henshall looked like cornering the market in serious and comic flaxen-haired flawed characters, but it was only with Primeval nearly a decade later that he became a household name.

The tall, intense actor does not want to be boxed in by television. “I like to do a play a year. I didn't do one for six years once and it was terrifying to go back.” The Last Days of Judas Iscariot is definitely an irresistible project, relocating the trial of Christ's betrayer to contemporary Manhattan and featuring figures such as Freud and Mother Teresa taking the stand and debating whether there is any chance of redemption for Judas. Guirgis's previous play, Jesus Hopped the “A”Train, also featured believers and non-believers in a modern milieu, so there may be a theme here.

Rupert Goold, who is directing, is no slouch either when it comes to location-shifting controversy, having directed Patrick Stewart in the RSC version of The Tempest set on an Arctic island. The cast is pretty good too, with Joseph Mawle, just seen as Jesus in BBC One's The Passion, now switching team colours to play Judas.

For Henshall, taking the part of the full-on Beelzebub was an absolute no-brainer. “I really liked the play's moral ambiguity, because I'm a little that way myself. It doesn't buttonhole you with what a supposed version of what good and evil, right and wrong, is, which is more accurate than society would have you believe. Also I love the Almeida. I live five minutes away so I can walk home. I never really think of the Devil as bad. I never see whoever I'm playing as a baddie; I'm always empathetic with them. The Devil says: ‘I don't believe in good and bad, I believe in truth.' That's what the play is saying. Both God and the Devil are there if you want them.

“The thing about Judas is that unless he can forgive himself, it doesn't matter what God does.” Could the play cause offence? “It's a very spiritual piece, which offers hope and not false hope. It is quite human, not something from on high.”

That all sounds jolly reasonable, but ask Henshall his own views on religion and he explodes, virtually thumping his fists through the table. “I loathe religion of all forms. I think the Church especially is responsible for more pain, more suffering, than any other thing on the planet. I would have those people involved in it shot.”

It is difficult to see why he reacts so strongly. His parents were religious but far from hardcore. Henshall simply took it all in and then rejected it. “I was taken to Sunday school as a kid, but there was no pressure, no pivotal moment. I never had it forced down my throat. It was more like just sitting back and hearing everyone spout a load of bollocks that really got to me.” If he can channel a tenth of that anger into his performance, this will definitely be a Lucifer worth seeing.

I remember Henshall being pretty intense eight years ago when I interviewed him when he was playing a lovelorn long-haired Levin in Channel 4's version of Anna Karenina. He was fine discussing his work but bridled when asked about his private life or if he'd consider doing shampoo adverts to show off his cascading locks. Today he seems mellower and less inclined to take everything so seriously. Perhaps this is down to his shorter, less conspicuous hair; it may also be because he is more successful and in a happy relationship with the Croatian playwright Tena Stivicic. Their meeting was pure romance: “I went to see a play she'd written and I spent most of play looking at her, not knowing she was the writer. Then a couple of months later we met on a blind date and have been together ever since.”

These days Henshall usually saves his intensity for the stage or the poker table, taking the card game far too seriously. He learnt to play while researching a role and was hooked. “I don't play with friends because I don't like the idea of taking money off them.” Instead he goes out to clubs and plays regularly in tournaments. “I'm competitive at whatever I do and I can't do sports since I did my knee in, so I play cards instead.”

Henshall can afford to lose a few quid these days, having made his name in Primeval. After two series, with a third to be filmed later this year, he has learnt to live with being stopped in the street by small, excitable boys. “People say nice things so I don't mind the attention.” If he ever gets too big for his boots he only needs to look at the sales figures of his Professor Cutter action figure scale model to be brought down to earth. “I'm quite sure it is not the bestselling figure from the series.”

Primeval is due to air in America later this year and, inevitably, he has already been to Hollywood for meetings. Less inevitably, this man with a distinctly serious streak enjoyed the experience, warming to both the climate and the businesslike work ethic. “I loved it. I got a US manager and will go back when Primeval goes out. It looks like they spend money on stuff out there whereas here we don't like giving money to people. Here we go: ‘Here's a fiver. If you can make it for that, fine. If you can't, f*** off!' They invest in success. When we have a success, we try to make it quicker and cheaper and worse.”

It is funny how a career can pan out. When he was playing Biff opposite Brian Dennehy in the acclaimed 2005 production of Death of a Salesman at the Lyric, did he ever expect to be a Saturday night tea-time regular? “I'm usually the guy on at 10pm on Channel 4, but I'm fine about it...” There is a long pause while he thinks, before suddenly erupting with another volcanic tirade. “It [Primeval] is old-fashioned entertainment and, if there is more of that in the world, we might get TV back from the clutches of terrible people like Simon Cowell and Sharon Osbourne. Their shows are one step removed from lions and Christians.” Never mind Satan in Manhattan. As far as Henshall is concerned he's alive and well and on X Factor.

The Last Days of Judas Iscariot is at the Almeida Theatre, N1 (020-7359 4404; www.almeida.co.uk), from Friday

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