When John Travolta is in a bar and the Bee Gees' Night Fever comes on, the actor always experiences a moment's trepidation. "Of course it makes me smile," he says. "And it makes me feel very validated. But I also have this fear that people are going to expect me to get up and dance. Now I don't mind dancing, but the days of wanting to be circled by people while dancing? They're gone." At 59, the star of Saturday Night Fever, Grease, Pulp Fiction and Face/Off has handed his dancing shoes down to his three-year-old son, Ben. "He's a dancing boy: so fun - so adorable. He sings and dances and plays with his planes [Travolta flies his own, including a Boeing 707] and it's just so interesting to watch how these things are passed on from one generation to the next."
It's four o'clock on a Saturday afternoon in sub-zero New York and Travolta is in the mollified, expansive mood that follows an afternoon nap. Or maybe he's always this warm and unguarded, in which case those who have booked tickets to see the New Jersey-born actor interviewed live at London's Theatre Royal on Valentine's weekend are in for a good night. Travolta's a talented anecdotalist, dipping in and out of the significant periods of his life with relish and nostalgia - taking himself easily back to his childhood in Englewood, an inner-ring suburb of New York, where, the youngest of six, he was raised by a second-generation Italian-American tyre salesman and an Irish Catholic actress turned English teacher. "I always feel that people who gain a certain level of success are indulged from the beginning in a way that others aren't," he says. "I think that they are either loved and admired from the get-go, or they haven't been and they're demanding their moment."
It's easy to see how a young Travolta - with his cleft chin and sensuous Italianate features - might have been indulged as a child. Certainly his acting ambitions were never about making himself heard above his siblings, he assures me. "I could take out the garbage and I would be heard," he chuckles. "I never needed to do a lot to get attention, so my performing honestly came out of joy." At 12, his mother put him in the Frank D Gilroy play Who'll Save the Plowboy? and encouraged him to consider acting as a profession, but it wasn't until he turned 17, after two years of summer theatre, that he got his first professional gig with an Equity theatre group. "I remember the director said: 'You know, other people have come in here and auditioned but none of them had the joy in performing that you have. It's contagious - and we're hiring you for that reason.'?"
Travolta's "contagious" quality could be condensed down to a boyish naturalness that he still possesses today, after four decades at the top of the business. As well as his looks and dancing ability, he exuded an unschooled sincerity, from his early years in Urban Cowboy and the Brian De Palma thrillers Carrie and Blow Out, through to Nora Ephron's comic hit Michael. Travolta, it turned out, could do comedy, morphing from a light-hearted romantic hero in Amy Heckerling's Look Who's Talking to a dark-humoured, morally corrupt villain in Barry Sonnenfeld's Get Shorty and Quentin Tarantino's Pulp Fiction.
Once the actor's un-nuanced prettiness filled out into something more solid, he re-emerged as a Hollywood hard man, scoring leading roles in thrillers such as Swordfish, Domestic Disturbance, Savages and last year's Killing Season. "I always knew how lucky I was," he tells me, when I ask whether being nominated for an Academy Award for his role as the sinuous-hipped Tony Manero in Saturday Night Fever, aged 24, and becoming a global sex symbol overnight went to his head. "So actually I was quite humbled by success when it came." It did, however, he concedes, make the next 10 years his hardest. "I felt like the decade between 25 and 35 was my most difficult. Then, once Pulp Fiction came into play, I was being offered the most wonderful roles again."
Even during his hiatus, Travolta was always "a glass-half-full man", he says, "an optimist by nature". He took care never to lose the joy that director had seen in him as a 17 year-old - a joy that should by rights have been drained away by now, not by the various celebrity torments he has been subjected to over the course of his career - the lawsuits, extortion plots and unending media speculation about his sexuality - but by the series of tragic losses the actor has experienced in his lifetime. First there was his long-term girlfriend Diana Hyland, whom he met while filming The Boy in the Plastic Bubble and nursed through a long battle with breast cancer until her death in 1977. Then in 2009 Travolta and his wife of 23 years, actress Kelly Preston, lost their autistic son, Jett, after the 16 year-old suffered a seizure and hit his head on the bath at the family's holiday home in the Bahamas. "I'm probably less terrified of death than your average fellow now, because people so near to me have suffered before their time and I just feel that if they can do it, so can I," he admits today. "The edge - the panic that most people feel - has been taken off death for me. I almost feel like it's disrespectful to fear it when others have been able to do it."
He and Preston try to keep their son's memory alive for themselves and their 13-year-old daughter, Ella Bleu, by talking about Jett freely, he says. And he holds on to the traditional Catholic belief that "a person's soul lives on forever", despite having been converted to Scientology in 1975 by the actress Joan Prather on the set of The Devil's Rain in Mexico. "I wasn't well and she gave me what's called 'an assist'. I got well very quickly after that, but I mean 30 minutes later." Travolta was intrigued enough to sign up to a course on his return to Los Angeles, where he found that Scientology "used everything I had always known to be true and provided really workable solutions. He," he says, presumably referring to science-fiction writer L Ron Hubbard, "has done the heavy lifting for us and worked to hone down what works into a technology that helps you attain the things that you are looking for in a religion."
If it weren't for his beliefs, Travolta assures me, he could never have got through the period following his son's death. "Oh my God," he says, and there's a crack in his voice even now, "I don't know what I would have done if I hadn't had the support of Scientology. I don't think I could have got through it. They were with me every day after Jett died. They even travelled with me when I needed to get away. And for a solid two years it was like that. It was only in the second year that I started to take a break of a day or two just to see how I was doing on my own."
I wonder whether, well before Jett's death, Scientology had become a crutch for Travolta - and Tom Cruise after him - to lean on when the pressures of fame became too much. A private man who remembers a time when showbusiness was about "ability being valued over getting attention for no particular reason", Travolta is bemused by today's reality television culture. "I always feel like a terrible snob when I say this," he says, laughing, "but I would be embarrassed to be famous for not doing anything. Fame is already a tricky thing but to have it when you have no particular ability is a strange thing, I think." It's about a modern cult of "voyeurism" he says. "What do they look like going shopping? What do they look like when they go on a date? How are they with their children and when they eat? What else can all that be but voyeurism?" To think that people invite that scrutiny into their lives through social media is anathema to Travolta, who once described the seclusion famous people are forced into as being like "a celebrity prison".
Of course Diana, Princess of Wales, was the ultimate victim of that, and when the two of them had that famous dance at a White House gala dinner in 1985, Travolta felt that it was "two victims of it dancing together". "There really was something lovely and girlish about her," he remembers, "and I felt that I had taken her back to her childhood, when she had probably watched Grease - and for that moment I was her Prince Charming." He wasn't nervous, he says, "because I'd seen her dance with Charles beforehand so I knew that she was strong. But she looked like she was leading [him] and because I knew that the world was watching I thought that I really needed to give her certainty that I knew what to do. I put my hand in the middle of her back, brought her hand down so that it wouldn't be so high and gave her the confidence that we would do just fine," he smiles. "She was the real deal. And I think that was partly because she followed the rule that 'it's OK to be important as long as everyone else is equally important'."
One gets the sense that Travolta lives by that mantra. He's gentle and courteous throughout our interview; he's also unafraid to sound vulnerable. "I don't love the idea of turning 60," he admits of his birthday later this month, "so I had hoped to keep it under the radar." Having feared that he might never act again after Jett's death, he now has a series of tough-guy roles lined up, first in heist movie The Forger, then as John Gotti Snr in a 2015 biopic about the mobster. He won't really "close the chapter on playing villains", he says, until he gets cast as a baddy in a James Bond film. "I would love that. They're going a different way with their villain in this next film but I've spoken to Barbara Broccoli about it and she loves the idea, so that would be great."
His white, three-piece disco-dancing suit may now be a museum piece, but Travolta still likes to sing - particularly to his son, Ben. "I like to introduce him to songs in unusual ways. I recently told him about the fastest aircraft in the world - the SR-71 Blackbird - because he loves planes like his dad, so I said: 'You know, Ben, there's even a song about the Blackbird.'?" Here Travolta breaks into a few lines of the Paul McCartney song - that unmannered high tenor still surprisingly affecting, all these years on. "The nanny was listening and do you know what she said? 'I didn't know that Paul McCartney wrote that about the SR-71.'?" He's still laughing softly to himself when we say goodbye.
A Conversation with John Travolta is at Theatre Royal, London WC2 (0844 412 4660) on February 16