In The Fixer he plays hitman John Mercer, a buttoned-up soul with a piercing gaze who shoots people for a living. Yet, though Buchan undertook a rigorous training regime to ensure his athletic 6ft 1in frame looked good in a Die Hard vest, The Fixer - by Ben Richards, who also wrote Party Animals - is no brain-dead shoot-em-up. Mercer is a complex character caught up in shadowy intrigue when he's mysteriously released from prison at the behest of Peter Mullan's maverick law enforcer.
Blurring the thin line between good and bad, The Fixer aims to keep the viewer guessing. 'The first killing you don't even see,' says Buchan. 'It's not about blood splatters all over the place, it's not about violence that's in your face - it's much more implied, it's all in your mind.'
Not that The Fixer is without its moments of light relief. Counterpointing Mercer's downbeat intensity is Jody Latham (Lip in Shameless) as Mercer's motormouthing ex-cellmate Callum. There's also a running gag in which Mercer says: 'You'll never see me dance.' It's a sly joke because tucked away on Buchan's CV is a certificate from the Ritmo Tropicale dance school in Ecuador. He can do a mean salsa or merengue if required.
'Spanish was one of my languages at university, so I spent time in Ecuador. And they don't have discotheques, they have salsatheques, so I learned salsa from a little lady who was about the size of Ronnie Corbett - if Dirty Dancing comes calling, I'll be there.'
Then he has to dash back to rehearsals. 'But you're the lead, don't they have to wait for you?' I ask. 'Oh yes, I suppose I am!' He may have had all the luck, but Andrew Buchan is keeping his feet firmly on the ground.
The Man Who Had All The Luck previews from Thu, opens Mar 5, until Apr 5, Donmar Warehouse, 41 Earlham Street WC2, Mon to Sat 7.30pm (Mar 5 7pm), Thu and Sat mats 2.30pm, £7.50 to £29. Tel: 0870 060 6624. www.donmarwarehouse.com Tube: Covent Garden The Fixer is due to start on ITV1 on Mar 10