The sun is shining on a spring afternoon in Venice Beach. Salty Pacific air sharpens the breeze. Roger Sterling, senior partner of Madison Avenue's top advertising agency Sterling Cooper Draper Pryce, rolls up to the cafe on his push bike, silver hair gleaming, posture ramrod straight. For once, he is not wearing a three-piece suit.

That is because he is not actually account executive Roger Sterling, rascally quipster and LSD enthusiast. He is John Slattery, the actor who has made dapper, unflappable Roger the most beloved character of Mad Men's five seasons, with his droll zingers and talent for saying the perfectly wrong things at the perfectly right times. Even in his civvies (windbreaker, plaid shirt, Ray-Bans), Slattery passes as preppy, weekend Roger. Nantucket Roger. It's that distinctive silver hair, that fox-alert bearing. Surely he's recognised wherever he goes?

"Yeah ... a fair bit," he answers in a measured, thoughtful way, a hint of his native Boston accent flattening out his speech. "They yell out – it just happened a minute ago on my way over here. Somebody yelled out 'Roger!'" He mimes a weary, bemused wave.

There are only two reasons why a person lives in Venice Beach: they're either a gang member or a surfer. I take a stab and guess that Slattery is the latter. "I am a surfer. I do that a lot. I could not do anything else and just do that all day. I just came from there."

Slattery has been surfing for 15 years, but his Mad Men success means that he has had to add some evasive manoeuvres to his technique when some kook in the swells screams "Roger!" at him.

"I usually paddle away from that. I kind of go out there to get away from that. It's not very social. I mean, it is if you're with a group of friends, but otherwise it could be pretty quiet out there. You see dolphins all the time – I saw them yesterday jumping around."

With the sixth and penultimate season of Mad Men in the full frenzy of production leading up to the premiere on 10 April (7 April in the US), Slattery is clearly savouring his limited dolphin time. It's well known that the cast are sworn to secrecy about plot developments, and the actor strains to impart a smidge of intel about series six without being in breach of contract.

"This season gets more complicated, because the times are going to get more complicated," he reveals. "Roger falls behind the centre of this youth movement and the emphasis starts to shift again. Everyone struggles with change. People's sense of mortality starts to enter the picture. Every decision becomes more critical."

As Mad Men's 1960s bloom into "the 60s", Roger's retrograde masculinity is becoming increasingly irrelevant. The culture clash is summed up in the contrast between the character's buttoned-up three-piece grey flannel suit, and his office's op art graphics and Eero Saarinen furniture. Losing his sure touch has opened Roger up to unexpected evolution, including taking LSD in the last season.

But evolution is not without growing pains. I remind Slattery of a scene in which Roger, humiliated by his younger rival Pete Campbell, whines: "When is everything going to get back to normal?"

Slattery nods. "Right – the 50s."

But ostensibly Roger's living the dream: he's at the top of the heap, he's got the beautiful wife. And yet there's a hollowness. "Sure. That's the American dream – you get what you want and then you don't want it any more."

As Roger loses interest in the things that have defined him – women and work – the character becomes more complex. It's intriguing that this 50s throwback is the one making overtures towards late 20th-century counterculture.

Slattery warms to the topic like an academic with his doctoral thesis.

"I would argue that Roger, though he's of a certain age and the country's emphasis is going in the other direction – 1967, 68, whenever we start back up – it's obviously a very youth-oriented situation ... but he takes acid, he's trying to shake it up. He is looking for something. He isn't ready to give up yet. He isn't ready to throw in the towel."

It's a testament to Slattery's talent that these developments don't feel forced or insincere. How does he arrive at his acting choices? "I give credit to [show creator] Matt Weiner for the trajectory. Whatever I do is basically to add to what's on the page. It's so well-written that you can play it with intention and bring detailed specificity to what's already written."

I gather that improvisation is not encouraged on the Mad Men set. Slattery jumps in emphatically: "It's a dirty word. A dirty word ..."

What happens if someone fires an extra salvo of spontaneity? "There isn't any. It gets excised. I mean, accidents happen, and they've been used in the past, but the show is precisely written."

He does get all the best lines, though. "I do get good lines, but I'd give up a couple of jokes for an apartment. I think Roger's the only character where you don't see any of his home life. And that's probably ..." he pauses, then continues somewhat unconvincingly, "a smart thing. But sometimes I'd like to see more of his life. But the thing about Roger that props up his world view is that you don't see too much of it. You see him at work, you see him at certain social situations, but you don't see a hell of a lot of Roger waking up in the morning and getting started, or doing the dishes. Not that he probably does his own dishes anyway. So he gets good lines, but everybody has ... a pretty full plate. Pete Campbell is a great character."

It sounds as if Slattery wouldn't mind a serving of weaselly Pete's dark torment on his own plate. "Yeah, I look at some of those stories and I think ..." he trails off. "Pete Campbell, I've always loved that character. People like to pigeonhole him, but I think he's a really complicated, great character."

What's it like playing rogue-to-rogue against Jon Hamm's Don Draper? With the competitiveness that comes across in the show, I wonder if there's a little bit of positioning between the actors themselves.

"No," he says firmly. "We have a great time working together – we always have, from day one. I always enjoy those scenes – we just did a couple the other day. Their relationship has evolved, too. They've watched each other go through good and bad periods, and I don't think they compete with each other, I think they appreciate each other. They really are friends."

Female viewers particularly dote on Roger. A friend spontaneously combusted with "Mmmmm!" when she heard I was meeting Slattery. Another said: "He's pretty. Can I come along and pet him?" Even an acquaintance's three-year-old daughter loves him. Erm, what does a toddler see in Roger?

"She talks about how she likes the way he looks: his face, his hair, his suits," her mother tells me. "But what she really likes is the way he barfs."

Ah, the infamous vomit scene from the first season, payback from Don after Roger hits on his wife. After a boozy, oozy lunch, vengeful Don rigs it so that Roger is forced to climb a skyscraper's worth of stairs in a rush to meet important clients back at the office. The combination of martinis, oysters and physical exertion results in an impressive ululation of sick across the clients' well-shined shoes. What were the logistics behind the scene, projectile-wise?

"It was Campbell's soup," Slattery divulges. "The special-effects guys rigged a tube up my pant leg, over the side of my ear, and to the side of my face. When we got to that shot, it was a countdown: three, two, blurrghhh!" Slattery's face creases into mirth. "Sometimes they'd push more out, sometimes they'd push less out. It's all make-pretend. It's funny, someone just sent me this gif." He scrolls down his smartphone. "Here it is!"

Slattery cues up the gif, with Roger eternally doubled over, spewing a Möbius strip of barf. "It just doesn't stop," he marvels. "It just goes on and on."

His family must be so proud. How do they feel about his Mad Men success? "They are sort of used to it at this point," he shrugs. "They enjoy the show. My mother likes it. She signs my name to pictures. For friends. She'll get out some Xeroxed copy of some old picture and sign my name to it."

His mother is a forger, I observe. "She's a forger," he agrees. "Among other things."

Is she in charge of the memorabilia collection of Roger action figures? "I do stow all that stuff away somewhere … the Roger dolls and the posters and the pictures and stuff that comes every year with each new press juggernaut," he sighs. "But by the time you finish the series, the last thing you want to see is another suit."

Does he know where Mad Men is headed? "I don't know where it's headed next week."

Mad Men season six: Bertram Cooper (Robert Morse), Pete Campbell (Vincent Kartheiser), Joan Harris (Christina Hendricks), Don Draper (Jon Hamm) and Roger Sterling (John Slattery). Photograph: Lionsgate

It's being written as they shoot? "Yeah. We have no idea. I mean, we have a read-through of the next script tomorrow and I haven't even read it. I've been directing the one that we're doing now, so I haven't had a chance. I've directed about five episodes now. It's complicated and good and difficult and time-consuming. It's creatively great, fantastic." As he enthuses about directing, Slattery's clipped Boston accent further asserts itself. "I've always wanted to do it, and it was a little late to go to film school. And it has been film school for me. I would like to continue after Mad Men finishes – make movies, maybe direct plays. I like making creative decisions. I'm never short of an opinion, and in most situations, it's not your job to make that decision. So I like the opportunity to make the creative decisions."

Slattery, one of six brothers and sisters, was "always a film fan and a visual person". He attended Catholic University in Washington DC in the early 80s heyday of the city's seminal punk and new wave club, the 9:30. I ask if he has seen a few wild nights at the "dirty thirty". "I wasn't sophisticated enough to have frequented the 9:30 Club. I wasn't cool enough in the 80s."

Was he a nerd? "I wasn't even a nerd. I was just hanging around smoking pot."

Over the years, Slattery has popped up in films (including The Adjustment Bureau) as well as in recurring roles in Desperate Housewives and Will & Grace. In Sex and the City, he played a politician who memorably campaigned for Carrie Bradshaw to urinate on him.

Slattery describes himself as "an enthusiastic Anglophile", too. "One of the things that made me want to be an actor was Derek Jacobi's Hamlet. One of those BBC productions with the old muslin flats shaking every time anybody walked near it. And I'm a big Downton Abbey fan. But I don't know about that last episode, with what's-his-name dying in the car. It was a little ..." he trails off disapprovingly. "I was talking to the television. I guess that's what they want. They want you to be talking to the television: 'You've gotta be shitting me!'" He brightens at another thought. "Ricky Gervais is great."

Interesting – many Hollywood types were vocal about finding Gervais too cruel when he hosted the Golden Globes. "At least he was funny! I don't know. All those award shows have gotten so weird. They used to be a kind of in-crowd, this is our industry acknowledging the good work of our peers, and now they're like the Super Bowl."

Has he prepared "winner" and "loser" faces for the awards show cameras? Slattery barks a short guffaw. "I have the loser face down. I generally walk around with the loser face most days. I've perfected it. The winner face – I have no idea what that looks like."

Maybe it's similar to his orgasm face? "I hope not. We'll find out. Or we won't. Most likely we won't. Most likely I'll live with my loser face."

And what is his loser face? "I don't know. You'll have to check the tapes."

Mad Men season six starts Wednesday 10 April at 10pm on Sky Atlantic