Seth Rogen is reliving the ordeal of having to go torso to torso with the immaculately sculpted midsection of Zac Efron, his co-star in new "family v frat house" comedy, Bad Neighbours. "Really, no one but no one should have to stand shirtless next to a shirtless Zac Efron. I wouldn't wish that on my worst enemy. I mean, look at the guy: it's like gay scientists designed him in a lab."

Bad Neighbours pitches young married parents Rogen and Rose Byrne against a riotous college fraternity, led by Efron, which buys up the house next door and unleashes a 24/7 party apocalypse on them after they call the cops on one of their unhinged all-night bacchanals. It's a measure of how long Rogen has been around now that when his career took off in earnest seven years ago, he would have been playing the fratboy. "It was honestly fun to be the older guy in the movie," he says. "It was kind of the best of both worlds. I didn't have to play someone who was, like, in some drastically arrested-development situation, but at the same time I'm not merely the straight guy, I get to be ridiculous and funny too. Which is nice."

I last met Rogen seven years ago, just as Superbad – which he also co-wrote with his regular partner Evan Goldberg – and Knocked Up, his first starring role, were about to firebomb the summer 2007 box office. We enjoyed a sweary and scatological two-hour brunch at Jan's Coffee Shop in West Hollywood, and no one bothered us. Seven years later, Rogen hasn't changed a bit from his gurgling stoner variation on the essence of Fozzie Bear, but the infrastructure around him has, likely because he'd simply be mobbed by fans in public today. We meet in the more factory-like atmosphere of a busy press junket, with Rogen shunting back and forth between print and TV grillings all day.

Zac Efron and Dave Franco in Bad Neighbors. Photograph: Glen Wilson

So my biggest hope for this story, I tell him, is that the intervening years have turned that hugely agreeable young man into... "like, the biggest fucking egomaniac Hollywood asshole in the stratosphere? 'Cos, yeah, that has totally happened!" he chuckles.

Not so at all. Just like the younger version, there really is nothing tricky, moody or vain about the household-name Seth Rogen. This despite having fulfilled my prediction that he and his many friends from what he calls "Judd's World" (as in the director Apatow), would soon entirely displace the then-comedy establishment; and despite having written several more hits with Goldberg, including two, This Is The End and the forthcoming The Interview, that they directed together as well. Zillions of dollars richer now, he's still just like the cool, funny, friendly nerd from upstairs who drops in occasionally to smoke you out.

And he's still a pothead, albeit now a married one. Does he have the inside line on the really kine buds? Like that primo government weed Wes Bentley sells in American Beauty? "Nah, man, I get it from the clinic just like everyone else. I have the prescription and everything. Dude, I wish there was that super-strain One Percent Weed!"

Bad Neighbours reminds me we're overdue for a good frat-house comedy, with the joke here being at the expense of the doting new dad who secretly wishes he could still party with abandon. "I know the writers really well and Brendan O'Brien has two kids and Andrew Cohen's wife is pregnant, and I'm married and so's Evan and so's James Weaver, the producer, so we are all just getting to that age. The idea of struggling with not being able to do all the fun shit they could do before they had babies, that just seemed original but real. We always need something that we heavily relate to. Even if the story is stupid, we need something that's personal enough to make us care and be able to work on it. I mean, This Is The End is ridiculous, but at its core it's very personal, it's about relationships and friendship; it needs to have emotions we can relate to."

There have been some ups and downs for Rogen in the last seven years: the inevitable mini-backlash at the stubborn primacy of the Apatow/stoner/manchild tendency in American comedy (I'm not complaining), and the occasional movie misstep. These have mainly been those forays out of his R-rated comfort-zone and into the trickier realm of PG-13, in his superhero movie The Green Hornet and his oddest outing, The Guilt Trip, with Barbra Streisand as his mother.

"It doesn't work for us, PG-13, because our movies like to in some way be pushing the envelope, edgy, but more than anything it's about being able to do what's funny to us in the moment," he says. "And those are two experiences where we would come up with funny ideas and we couldn't do them because of the rating. It was the most frustrating thing; it was like fighting with one hand behind your back. We've tried several times at PG-13 and it's just not as much fun while we're making it."

We talk briefly about Rogen's sensitive dramatic role in Sarah Polley's exquisite Take This Waltz – "She called me, I was amazed and flattered" – before seguing into talking about how Canadian he feels these days, in the age of Alice Munro's Nobel Prize and Toronto mayor Rob Ford's drunken pratfalls. "Rob Ford is like Chris Farley. But if you go to Toronto it's actually a very well-functioning city! People like him, it's really bizarre, and now he's running for office again! That being said, smoking crack on camera isn't the savviest political move of all time, but he seems to be doing an OK job."

Rogen recently made a political detour of his own, appearing with a witty and charmingly presented disquisition on Alzheimer's disease before a Senate committee. He noted the pathetically thin attendance, and later called out one senator on Twitter for leaving halfway through. Rogen's mother-in-law was diagnosed with Alzheimer's around the time he was embarking on his film career. "And now she's in her early 60s and she's basically non-responsive. It's 100% fatal. And this is what's kind-of brutal about it: it could last five years, or it could last 25 years. But if you have it, it will kill you in the end. And it also takes down, or just slowly drains, everyone around the patient; that's why they call it a caretaker's disease, because it affects other lives. But it's not a 'trendy disease' and it needs far more attention from politicians." There's no emoting in his jeremiad, no grandstanding, just a sense of being tragically, perhaps helplessly, pissed off about the whole nightmare.

But a man must work: next up for Rogen is The Interview, in which he and James Franco play a TV producer and his anchor who snag an interview with their biggest fan, Kim Jong Un, and are asked to poison him by the CIA ("And we're like, 'We'll try!'") And then there's Sausage Party, an adult computer-animated movie about a grocery. "I'm a sausage, Jonah Hill's a sausage, Michael Cera's a sausage, Kristen Wiig is the bun – like, the love interest – Salma Hayek is a taco and Bill Hader is guacamole," he says. "It'll be like a Pixar movie… but with fucking!"

Like I say, the kid hasn't changed, not one bit.

Bad Neighbours is in UK cinemas from 3 May

• This article was amended on 2 May 2014. An earlier version referred to Alice Walker as having won the Nobel prize rather than Alice Munro.