Jason Reitman knows he needs to work on his drinking problem. Specifically: develop one. It's a long road ahead. He was an abstemious teen, an undebauched student. At 34, he's yet to try a gin and tonic – "two things I haven't been able to get my mouth around" – but thinks he maybe should. "I'm a total cliche – the compensating for growing up a rich kid in Beverly Hills, the compensating for being the son of a famous director [Ghostbusters' Ivan]. The one break is I don't have an alcohol or drug addiction. Then I would be the complete picture. And I think it's beautiful that we're all cliches, I really do."

He's been making progress. "Three years ago, I started drinking, and I tried pot for the first time two years ago. And then I found scotch and I was like: wow. This is so lovely." He leans back: game, playful, perhaps a touch jetlag mad, in London just before Christmas. "The feeling is so lovely. I like that it has a weight to it. Rather than vodka which kinda attacks with a hammer."

There's a certain inevitability to the way Reitman – four Oscar nominations by 32 – skipped the kiddier end of the wine list and went straight for the tough stuff. Since his whisky epiphany, he's tried ryes, got into Maccallan and Oban, Laphroaig and Talisker ("the roughest thing I've tried; really peppery"). Like a curry? "Oh, I can't eat curry. But maybe that'll come next. I finally found an Indian food I liked: butter chicken."

Statements like that give one pause for thought as to Reitman's ultimate potential as a hell-raiser. But the lead in his latest movie, Young Adult, is nothing if not a convincing drinker.

Mavis Geary (Charlize Theron) is a teen-fiction hack who drowns daily hangovers in Diet Coke straight from the two-litre bottle. From her 4 Non Blondes mix tape to her Hello Kitty nightie, she's stuck in a mid-90s loop,  when she ruled supreme as prom queen in small-town Minnesota. And it's there that she returns from a sterile, semi-successful life in the city, in the hope of winning back a former sweetheart, Buddy (Patrick Wilson) – now happily married with a baby. It's surely a doomed mission: Mavis is a mess, vile even to her one ally (Patton Oswalt's mordant Matt). She's a writer who can't read people, who chucks herself confidently into situations then thinks that the ensuing gasps are of admiration. And it's this perverse courage, the fruit-loop naivete, that means you find you like her.

Young Adult is a conscious jump into the dark from a man who knows his career can cope with a little lukewarm box office. After three films that established a career (Thank You For Smoking in 2005, Juno in 2007 and 2009's Up in the Air), this one, he calculated, could "service art and art alone". The sales strategy followed suit – no festival circuit, but drip-fed to fans with pop-up screenings and bespoke posters. It's a sharp move: Young Adult repays percolation. Its pessimism and humanity mushroom long after a final act that slaps out three scenes of increasing unease, then refuses to deliver redemption, even after self-realisation.

"If Mavis became a different person people would just watch the movie and go: that's right, people become better people and I'm gonna continue becoming a better person and life goes on and that's great. Hopefully that's so jarring that the audience feels it for the days after, and maybe it becomes a moment that acts as a mirror and reveals something about their own character."

Some have been duly unsettled. Partly on account of Young Adult being Reitman's and Diablo Cody's follow-up to Juno, a much sweeter comedy adored by press and public (Fox Searchlight must have been chuffed, too, with the $231m return on a $7m budget). Those expecting a reprise have been surprised by the little vial of cyanide in their orange tic-tacs – good and tart, but an acquired taste. One test screening card for Young Adult bore the comment: "I don't know why Jason Reitman wants me to feel like this."

Does he ever fret he's gone too far? No – he loved that card. "I'm not worried that my film is gonna have some negative effect on people's lives. With the amount of noise out there you can only hope to cut through so much."

Plus, it's this sort of manoeuvre that gets him going. "What interests me is getting the audience to feel something very specific, usually for a moment. You do a magician's trick convincing them into the rhythms of a movie they're used to, only to twist them at the right moment."

Aesthetically, too, Young Adult is cleaner than what he's shot before, yet more deferential to the star and the screenplay (his programme of lo-fi live, classic script readings in LA suggests his priorities increasingly lie on the page). Gone are the splashy credits, the slick celluloid.

Young Adult is his Election; but while Alexander Payne looks to have got gummier of late, Reitman has been sharpening his teeth (for the record, he has some of the strongest-looking all-Canadian dentistry you'll ever see).

Born in 1977, he was raised on soundstages and edit suites, taken on the Animal House set at 10 days' old. There were bit-parts in front of camera – Wrong Kid in Alley in Father's Day, Kissing Boy in Kindergarten Cop (the clinch was cut short by Arnie with a machine gun). But he hesitated about signing on for the family trade – his mother, Geneviève Robert, is an actor – until a pep talk from his dad convinced him. (It's an exchange that echoed another conversation his father had, back in the 60s, with his own father, who wanted to steer him away from a career in sandwiches.)  

So, Reitman switched from medicine to English at university, shot shorts, including an especially successful one, Consent, about pre-sex red tape, co-written by his then wife, Michele Lee (they had a daughter, Josephine, in 2007, before separating last year). Out of college, commercial work paid the bills and stoked the artistic ambition. He twice turned down the chance to direct Dude, Where's My Car?, instead optioning Christopher Buckley's Thank You for Smoking. After that: Juno, then Up in the Air, another macho satire Reitman adapted himself (this time from the novel by Walter Kim).

Though none came with the sting of Young Adult, all three chucked curveballs with their feet in the mainstream. Smoking fogged your politics, Juno flipped expectations, Air ended in romantic bathos. All worried at the compatibility of happiness and domesticity.

Next is Labor Day, his own script of Joyce Maynard's bestseller about a single mother (Kate Winslet) who takes in an outlaw (Josh Brolin). A female lead, then, but no Cody. And a wider scope: until now, the key has been tone, not plot. "That's why I never know where my characters were before the movie starts or after it ends – it doesn't even interest me." Labor Day is different. "I'm not scared of doing drama. What scares me is that it's dramatised. It plays like a classic novel – it takes place over four decades, it needs to be more stylish in the way that it's shot. We'll see if I'm capable."

For Reitman, film-making is part self-test, part internal inventory. "I'm trying to figure myself out through my movies. Whether it's big stuff like what we're doing here, or little stuff like why aren't I happier? With every film I feel like I'm apologising for something. I feel I'm most successful when I'm looking for something that embarrasses me about my character that I'd like to expose."

The list, when pressed for, is long. "Selfishness, narcissism, being uncomfortable in your own skin, not feeling connected to the world around you, feeling dislocated from family and youth, having a strange relationship with your childhood – all those things feel really true to me."

Reitman feels a curious mix of ages. His voice fluxes between chewy drawl and vaguely nasal. He seems naturally happy, is good company, but with a bounce that could, you sense, be burst by external circumstance. White as well as black hair sprouts from beneath his beanie. That hat bears witness, he says, to his own development being as arrested as the rest of his age-group's. But he's wise to the possible contradiction. "Doesn't every generation feel like the one that's coming up behind them doesn't know how to grow up? I'm not sure if we're progressively getting worse or if your perspective shifts."

Presumably becoming a father jump-starts maturity? "I guess you can only feel responsible for something you can't walk away from, and you can never walk away from being a parent. But where once people felt they really couldn't walk away from marriage – and I say this as someone who is divorced, so I'm not exactly proud – now that it's so pervasive it probably helps to permeate this idea that you don't really need to grow up."

Anyhow, he says, "Growing up sucks, doesn't it? I understand why people wouldn't want to get old – but it'd be one thing if we became a culture obsessed with eating right, doing yoga, going to therapy and becoming at one with ourselves. That be great. But we don't do that. We seem to be obsessed with all the wrong ways to stay young." Why? "I'm just making complete guesses, but maybe because it's a lot easier."

Does he feel older than his years? "I've a bad heart; I'm probably gonna die young, so perhaps it's just it's the percentage of my life that I've lived so far. I really hope I see my 70s, though most people our age presume 80s are kind of a given and 90s are even probable. I think I have to adjust my lifestyle if I want to live longer."

More scotch? "That's the key, right?" He beams. "What doctors recommend."