Breakfast time in a Covent Garden hotel. Philip Seymour Hoffman is demolishing a bowl of cornflakes and complaining that the Oscar he won for his uncanny performance as Capote in 2005 has robbed him of anonymity. “Now, if someone looks at me it's 'cause they recognise me,” he grumbles in a bass monotone, “whereas before, it probably just meant I had food on my face or something.”

Actually, no one in the packed room seems to register Hoffman's sensual features — those hurt eyes, that woeful mouth. And he is, after all, a chameleon. The 41-year-old New Yorker has given us bruised losers (Boogie Nights, Magnolia), loathsome porkers (Red Dragon, The Talented Mr Ripley) and offbeat romantic leads (State and Main, Owning Mahoney). Just last year he was all over cinemas with three superlative but wildly different roles in The Savages, Before the Devil Knows You're Dead and Charlie Wilson's War. But he's in London as a theatre director, not a film star, so maybe he's giving off an offstage vibe.

Directing has been as much a part of Hoffman's life as film and stage acting since he joined New York's experimental Theatre in 1995. Indeed, his acclaimed LAByrinth production of Stephen Adley Giurgis's Jesus Hopped the “A” Train transferred to the Edinburgh Festival, then the Donmar in 2002, kicking off his international directing career. “I had a great time back then, and London is a city I know well in terms of its theatre,” he says. “In fact, it feels like just another city where I work now: I keep trying to remind myself to treat the place like a tourist, go to Tate Britain, take in a couple of shows, just walk around. But I forget.”

He's been tempted back by Riflemind, written by Cate Blanchett's husband, Andrew Upton: Hoffman has been friends with the couple since he appeared with Blanchett in The Talented Mr Ripley 10 years ago. “We didn't have a whole lot to do, so all three of us just hung out in Rome all the time,” he recalls. “Andrew became a good friend of mine. We think the same way, and some time ago he sent me an adaptation of a Bergman piece. We got together, did a few workshops and then he sent me Riflemind.”

The play tells the story of two brothers getting their once-seminal rock band back together, only to find that sex, drugs and rivalry die hard. Hoffman directed the premiere last year at the Sydney Theatre Company, which Upton and Blanchett jointly run, and many of his Australian actors are reprising their roles in London, with John Hannah and Paul Hilton taking the leads as brothers John and Phil. The play ushers in a new creative partnership between Trafalgar Studios, STC and LAByrinth, securing its position as an exciting new producing venue.

I ask Hoffman what it was in the play that spoke to him. “That's a conversation that never ends,” he says. “The longer I work on it, the more things it seems to say. It's about the complexity of relationships and the way that lives change whether we like it or not, and how hard that is for those who change and those watching them. It's about what it takes to create [art], the sacrifice that it takes and also the evils that are involved.”

Ah. Hanging over all of the dried-out and detoxed characters in the play is the fear that they will start using again. Hoffman himself hasn't drunk a drop since he went through rehab at 22, after graduating from New York's famous Tisch School, to deal with a dependence on booze and pills. Did this inform his view of the play? He gives a deep, sarcastic chuckle, hur-hur-hur. “Well, it was a long time ago, so it's not like I'm sat there going well, when I was in rehab…' ” he says, “but I bring my experiences along. And the biggest fear among some of the characters is not addiction but the fear that you have peaked in your life: that you don't have it any more, and the excitement of being young is behind you. That everything is old, everything is used, and this is now all you have.”

Surprisingly, Hoffman suggests he knows what this feels like. Having started out in theatre, he got his cinematic break in 1991 alongside Al Pacino in Scent of a Woman, and soon became the go-to guy for richly textured arthouse character parts who could also add ballast to Hollywood fluff. It's possible to see the graph of his career curving upwards towards Capote and the Oscar. Does he feel it's downhill from there?

“I wouldn't describe the Oscar as a peak, although for a while it's probably made it easier for people to hire me,” he says. “No, the peak of your career is in those moments when everything seems to come easy — the work, the imagination. You are aware of that artistic power. All artists have a fear of losing that. When I was unaffected by experience, when I was young and there was an idealism to it, I wasn't as fearful. Growing older, fear can creep in. And if you lose the thing that keeps your heart pumping, you lose the will to wake up in the mornings. That is one of the many things Andrew is getting at in the play.”

The diversity of his work keeps him fresh. Theatre acting offers at least as many tricks and traps for an actor to fall back on as film, he says, and during long runs — like his Tony-nominated Broadway stints in Long Day's Journey Into Night and True West — an actor can develop a form of repetitive strain injury. “It's like you're doing this all the time,” he says, smacking a fist repeatedly into his palm in illustration. Film acting, meanwhile, is immediate: “You learn to trust the moment if you know you've just got three hours to nail something and you can never go back and change it.” He has three typically diverse movies due out: an adaptation of John Patrick Shanley's religious drama, Doubt; Charlie Kaufmann's bizarre-sounding Synecdoche, New York; and Richard Curtis's cheery homage to pirate radio, The Boat That Rocked.

Directing Riflemind here gives him a break from the consuming process of acting, which he likens to “lifting heavy weights with your head”, but he has one regret about his current sojourn. He's missing his partner of 10 years, costume designer Rosie O'Donnell, and their son Cooper, five, and daughter Tallulah, two. “One of my kids is starting school this week, and we've got a third kid on the way in October, so my family can't be here,” he says. “That's the not-fun part of this job.” I ask if having children has made him less obsessively focused, less driven. “I don't know yet, but having children is an all-consuming thing,” he says. “You can't love anyone or anything more than you love your kids, and you think about them all day long.”

I order the bill, Hoffman steps outside for a cigarette, and I watch him, a hunched, burly figure puffing with fierce concentration amid the bustling pedestrians of Covent Garden. No one seems to recognise him at all.

Riflemind is at Trafalgar Studios until 3 January. Box office: 0870 060 6632 or www.theambassadors.com/trafalgarstudios.