It's impossible to miss the diamond engagement ring sparkling on Johnny Depp's ring finger when he arrives, after a quick cigarette break on the balcony, to talk about his new sci-fi drama, Transcendence.

The three-time Oscar nominee with a penchant for playing larger-than-life characters from Captain Jack Sparrow to the Mad Hatter also likes to make extreme fashion statements with his own wardrobe, and today he's wearing a purple rockabilly jacket, a grungy striped shirt and hole-ridden jeans that could signal homeless status if he weren't so recognisably Depp.

But the ring is new, and already well publicised, as he eventually acknowledges with a grin.

"Yep, that's the one that is too big for her, that's why I'm wearing it," Depp chuckles, referring to the rock he originally bought for his fiancee, actor Amber Heard.

Depp never formally announced his Christmas Eve engagement to the 27-year-old actor he met on the 2011 indie film The Rum Diary, but he now seems surprisingly chatty about it.

"The first problem was that I designed a ring for her and made some drawings but I didn't know what numbers mean in terms of diamond size and, when it came back to me, I realised that the diamond was the size of a human eye," he concedes a little sheepishly. "Then I got her this subtle one from an antique jewellery person in New York and it was too big, so it hasn't left my finger since."

Is Depp seriously marrying a woman who would complain her diamond ring was too big? He laughs. "It wasn't the kind of ring you'd feel comfortable wearing down the street to get a juice," he insists. "If she'd waved hello to someone, it could burn the retina in their eye!"

Depp, 50, first built a strong critical and art-house following in the 1990s after escaping his teen-heart-throb tag on the TV series 21 Jump Street by making films including Edward Scissorhands (1990), What's Eating Gilbert Grape (1993), Ed Wood (1994) and Fear and Loathing in Las Vegas (1998). He's so adept at disappearing into characters and accents that it's reassuring, when you meet him in person, to discover he's a somewhat quirky but extremely endearing character in his own right.

Self-deprecating and smart, Depp is also surprisingly open about why he's getting married again after previously vowing it would never happen. He's been married only once – from 1983 to '85, to musician Lori Anne Allison – but has been engaged three times, to actors Jennifer Grey and Winona Ryder and supermodel Kate Moss. He then lived with French model and singer Vanessa Paradis for 14 years and they had two children, Lily-Rose, 14, and Jack, 12.

"It's an interesting thing at a certain point in your life when you realise you are able to dedicate your life to one person," he says, running a hand through his mop of dark hair. "In a lot of ways, I am everything that she wouldn't want to be with, and she's everything that I wouldn't want to be with, because actors and actresses, it's not an easy deal.

"But she has a very good perspective on her career, and one of the first times I sat down with her, just talking after we met, I was astonished at how smart she is. She also has the ability to bring up the names of old blues guys, like Slim Harpo or Tampa Red, so there was really no way around it once I fell for her."

In Transcendence, Depp puts aside his more flamboyant characters to play Dr Will Caster, a brilliant artificial intelligence researcher working on a machine that will combine the collective intelligence of everything ever known with a full range of human emotions. After a shooting, his wife and fellow researcher Evelyn (Rebecca Hall) convinces him to upload his own consciousness to the machine. But his former best friend Max (Paul Bettany) is alarmed by his insatiable desire for power and technological breakthroughs, and begins to suspect that all of civilisation may be in danger.

"What I found most interesting about this film is that … it has this kind of classic feel of sci-fi but, as you get further into the story, you realise that the sci-fi is not sci-fi, it's actuality," Depp says.

"A lot of the technology that was being used in the film is really being used, and the ability to upload a human consciousness into a giant hard drive is actually just around the corner. So it leaves the arena of sci-fi and takes us into a promised projection of the future, which is very different and a little frightening."

The film poses questions about the future of a planet obsessed with technology.

"People have being fighting wars over, 'My god is better than your god,' for thousands of years," Depp ruminates. "I think it's appropriate to look at the way human beings worship technology and what that could mean for our future. Whether you're on the side of technology or ecology, a pacifist or an extremist, most people have very definite opinions, and the question of how far we should allow our dependence on computers to go is a question that needs to be asked."

Depp himself quickly confesses he's not a fan. "Every five minutes technology stumps my brain – just trying to answer a text with the crisscrossing thumbs and what not. Honestly, I'm that guy who travels with an acoustic guitar and a portable typewriter from about 1940, so I'm really the most base level in that way."

Born in Kentucky to a waitress and a civil engineer, Depp was the youngest of four. After his father left the family when he was 15, he began getting into trouble with drugs and alcohol until he found music. He dropped out of high school to become a guitar player, moved to Los Angeles and then stumbled into acting with a blood-soaked debut in the 1984 horror A Nightmare on Elm Street.

Depp has been nominated for three Oscars, for Pirates of the Caribbean: The Curse of the Black Pearl (2003), Finding Neverland (2004) and Sweeney Todd: The Demon Barber of Fleet Street (2007). Last year, critics prematurely wrote him off when his Jerry Bruckheimer western The Lone Ranger (with Depp in one of his most outrageous characters yet, the face-painted, dead-crow-wearing Tonto) tanked and forced Disney Studios to take a write-down of about $US190 million. It prompted his confession to the BBC at the time that he was considering retirement: "I wouldn't say I'm dropping out any second, but I would say it's probably not too far away."

But Transcendence is the first of three major Depp films this year. In the Rob Marshall musical Into the Woods, he plays The Wolf to Meryl Streep's The Witch; and in the comedy Mortdecai, co-starring Gwyneth Paltrow, Depp plays a degenerate aristocrat who works as an art dealer by day and a thief by night.

Later this year he will shoot the drama Black Mass, playing convicted Boston crime boss Whitey Bulger, and next year he's hoping the script will finally be ready for a fifth and final Pirates movie. Clearly rethinking the retirement, his more considered response today suggests he just may not want to give up his only escape.

"As you've probably noticed over the years, I have done my best to disguise myself because I'm super shy," he admits. "Just going on stage at the Golden Globes earlier this year to introduce a film had me rattling in my bones. But to play a character? I'm not afraid to do anything."

10 facts about Johnny Depp

1. He was allergic to chocolate until he was seven, but ironically made two films titled Chocolat (2000) and Charlie and the Chocolate Factory (2005).

2. Since he was a child, he's been a huge fan of motorcycle daredevil Evel Knievel.

3. Depp has a phobia about clowns, once explaining: "There always seemed to be a darkness lurking just under the surface, a potential for real evil."

4. He's listed in the 2012 Guinness Book of World Records as the highest-paid actor, taking in $US75 million between June 2009 and June 2010.

5. Depp still performs as a musician, playing lead guitar and drums for a cover of Carly Simon's You're So Vain, which appeared as a bonus track on Marilyn Manson's 2012 album, Born Villain.

6. Depp has a fossilised bug named after him: a 500-million-year-old ancestor of lobsters and scorpions, named Kooteninchela deppi in honour of his clawed character Edward Scissorhands, according to the Journal of Palaeontology.

7. In 2007, after Depp's daughter, Lily-Rose, was saved from an infection that began to shut down her kidneys, he donated £1 million in gratitude to the Great Ormond Street Children's Hospital and returned in Captain Jack Sparrow costume to read to other patients for four hours.

8. In 2012, while promoting his Indian character in The Lone Ranger, Depp - a part-Cherokee - was adopted by the Comanche Nation and given the Comanche name pronounced ''Mah Woo May'', meaning shape shifter.

9. Depp owned the famous Sunset Strip club Viper Room when actor River Phoenix overdosed outside the club (he later sold it) and also co-owned popular Paris restaurant-bar Man Ray with Sean Penn, John Malkovich and singer Mick Hucknall.

10. In 2012, he appeared in Paul McCartney's Queenie Eye music video alongside Natalie Portman, Meryl Streep, Sean Penn, Jude Law and other stars.

Transcendence is out on April 24.