Spike Jonze is looking to the future. "I kinda think, as I look around, that everything is slowly getting a little bit nicer. You can go on Nike's website and choose exactly what fabrics and colours and shapes you want your sneakers to come in. Everything in LA is; it's just an easy place to live in. The houses are nice, the backyards are nice, you got the ocean right there and the mountains behind you, there's an idealised easiness to the way you live and the whole environment.

"There's more good food here than ever before, better restaurants. In some parts of the world, like right here, you can feel the future coming toward you and a lot of it, in terms of things like convenience, availability, accessibility, beautiful design, ease of use, is getting better all the time." There's a slightly dreamy, wistful note to his words, as he lounges on a black couch in a back office, here in his present-tense Hollywood Hills post-production suite.

Jonze's new movie Her – his first as both sole writer and director (after his splendidly mind-boggling collaborations with Charlie Kaufman on Being John Malkovich and Adaptation, and with Dave Eggers and Maurice Sendak on Where The Wild Things Are) – is being sold as "A Spike Jonze Love Story."

And that's what it is: a love story, surprisingly sincere and heartfelt and affecting. But also a Spike Jonze love story, meaning it's set in a very near, slightly off-kilter future, in a squeaky-clean but still recognisable Los Angeles (complete with an unlikely fully functional public-transit system), which is augmented with shots filmed in other Pacific-rim capitals such as Shanghai and Tokyo. Says Jonze: "We wanted a kind of clean, calm, warm utopian future that has the same struggles and longings to connect that you find everywhere; feelings of isolation and loneliness. I wanted to tap into that a bit."

This love story – and here's the Spike part – concerns Theodore Twombly, a depressed writer played by Joaquin Phoenix (Jonze-ily enough, his job is writing heartfelt letters) who falls in love with "Samantha", his new computer and smartphone operating system, played, in voice only, by Scarlett Johansson. They have a rich, gratifying and fulfilling relationship. Until they don't. Just like a real relationship.

Ridiculous, right? You'd be surprised.

It's late September and I've just watched the movie, minus a couple of CGI digital inserts, at a majestic suburban house in the hills above the Sunset Strip which is functioning as the post-production set-up for films backed by Annapurna Pictures. The company's head honcho, producer and Oracle Corporation heiress Megan Ellison, lives in an even more sumptuous mansion right next door. The views of the city, all the way to the silver thread of the Pacific coastline, are million-dollar breathtaking. The others at the screening are the production and editing teams: they lock picture tonight after nearly a year in post-production.

Spike wants to know if I like it – he has a slight dazed look I've seen before in other directors at this stage of post-production: he's been editing so long he wonders if he can tell the wood from the trees any more. I loved it, I say. I've seen love stories between real human beings with bodies and bedrooms, and felt none of the rich emotions prompted in me by Her. I ask where the idea came from.

"The very tiniest seed came about 10 years ago when I saw this article online that said you can interact live with an artificial intelligence. So I went to the website, and I IM-ed this address, and I was like, 'Hi, how are you?' and I got responses like, 'Great, how are you?' And you can talk to it and tease it – not a him or her, it's just typing – and get a little banter going, getting mocked and so on. I got this sort of buzz thinking: this thing's actually keeping up with me. And then after a couple of minutes you start to notice the cracks and the flaws. Oh, this is a very cleverly written program, I thought in the end, but for those couple of minutes I got a very distinctive, tingly kind of buzz from the experience. The movie has a lot of large conceptual ideas holding it up, but most of all, I always wanted to make it a moving relationship movie – that was what I was most interested in."

It might have turned out very differently, though. On the set, Phoenix interacted live with Samantha Morton as digi-Samantha. "Samantha was a big part of it, and we've been friends for a long time. What happened in post was that we edited the movie for ages and finally realised that what Samantha and I had done together wasn't working the right way. It was a really hard realisation to come to, and it was really, really hard to tell her about it. So we brought Scarlett in during post-production, and we somehow got it to work." Johansson was working on Broadway in Cat on a Hot Tin Roof at the time, and Jonze grabbed her for rehearsals at any odd moment. At one point, Johansson said recently: "We were even rehearsing on Maggie the Cat's bed on stage."

Blessed with a naturally deep, expressive and erotic voice, Johansson turned in an extraordinarily deft and beautiful, yet totally disembodied, performance that won her the best actress award at the Rome film festival and has some critics calling for Oscar plaudits (though the Golden Globes have already excluded her from contention).

We're slumped on couches in one of the back rooms, and I'm gratified how agreeable and open Jonze is. He's kind of scruffily dressed in apres-skate attire, including an uncomfortably jaggy-looking pair of trousers that seem to be made from flour sacks. I know he has pranked a few journos in his time. He once asked an interviewer to join him on an urgent visit to someone difficult he had to deal with, and took him to Johnny Knoxville's house, who – pre-scripted by Spike – promptly declared violent rhetorical war on the stunned hack. Another interview on YouTube shows Spike at the wheel of his car in downtown LA traffic. Suddenly he goes all green, steps out of the car and emits a stupendous fountain of orange vomit, then sheepishly wobbles back into the driver's seat.

This is, after all, the man behind the Jackass TV show and movies. In fact, even as he is readying Her for lock-down, he's simultaneously dipping in and out of the production for the Jackass spin-off, Bad Grandpa, starring Knoxville as a fake 86-year-old granddad with a huge capacity for giving offence. Still, no pranks for me, not so much as a whoopee cushion.

I tell him that what I like about his movies is that they don't live in relation to other movies, as so much homage-heavy film-school product often does. They feel organic and hand-made, like carefully wrought, gleaming objects – for all their CGI interventions. But the Jackass movies have something in common with the earliest, oldest kinds of cinema, in which a camera was simply set up, and insane things were enacted before it.

"No doubt. We watched a lot of Keaton and Chaplin – and we stole everything from them we could. Abbott and Costello, the Three Stooges. To do a live-action Tom and Jerry cartoon was a big part of what we wanted. If you watch Jackass again you'll kinda see how much we plagiarised from Tom and Jerry – it's pathetic!"

But the Jackass approach – and, in a way, his whole approach to directing – also derives from Spike's teenage skating exploits with his friends back in Maryland. "We were the very first generation of teenage boys to have access to cheap, good, portable video cameras. That was just part of skating and hanging out, and the camera in that situation is an absolute inspiration – it just forces you to use it. You know, it could be for some epic ride – attach it to the board, maybe, or just goofing off and doin' pranks, like, hey, there's a shopping cart, climb on in and we'll push ya down this steep hill and into that big bush, and film it! It was just a part of skate culture to make videos like that."

Jackass just came about. One day they were sitting around bullshitting, next thing they knew, MTV had picked them up. "We thought, we have 20 minutes on national TV to do whatever the hell we want, as long as the lawyers approve it. We got no notes from them whatsoever, and we just made whatever we wanted. But then it came out finally, we had no idea ... Johnny Knoxville went from struggling to pay his rent to being on the cover of Rolling Stone in the course of like a month. It was like wow, what did we stumble upon here?"

Skateboarding cast a deep influence on Jonze's teenage aesthetics, but so did DIY punk rock and old school hip-hop. "I loved Fugazi, the DC hardcore band, because they always did everything themselves. They had their own label, and the CDs always cost nine dollars, the T-shirts always cost eight dollars, the shows always cost five dollars, no major label. Mainly I was loving their music, but also the idea of doing it yourself and not giving power and control over to big companies."

This conviction was deepened by his long collaboration with the Beastie Boys, for whom he made numerous music videos. "One of the amazing things they did when they got their advance for their second album on Capitol, Check Your Head, was they took their whole advance and a lot of time to build their own studio, Grand Royal, out in a neighbourhood called Atwater Village, really a little sleepy, 1950s small-town vibe to it. It was a kind of exploration of what it was like to make a record again, literally reinventing themselves for the third album in a row. They were always gonna make exactly the thing they wanted, even from the earliest videos I made with them, the label didn't get to have a say. It was all going to go through their filter first and last, and all be under their control."

It's five years since his last movie, Where The Wild Things Are, and he says it's likely he'll now do again what he did after that movie, which itself took five years to complete: cleanse his palate. "I took a year where creatively I wanted to just have an idea and go make it right away. I don't have to be making a movie just to be making one. I like to get to the point where I need to make a particular movie. In between times I make small movies, some animations – I did one in stop-motion felt back then. For a while I want to make things that are more like pencil sketches, while movies are more like large oil paintings. For a while I'm gonna think small."

The kid has quite a future. In fact, he just made a movie about it. Look to it.

More on Her

• Jonze talks about the movie in Toronto
• First look review of the film
• Oscar predictions examines its awards chances
• Golden Globes rule out Johansson
• What do you make of the trailer?

• Her is released in the UK on 24 January 2014