Clean-shaven and full of beans, Tom Hanks bounds into the room, wearing a suit and a smile that radiates polish.

"How are you, sir?" he inquires, pumping my hand. All he need say is "By Jiminy" and that gee-shucks image that has made him one of Hollywood's most likeable stars would be complete. This is, after all, Forrest Gump. Woody from Toy Story. And now he's good old Uncle Walt.

In this month's Saving Mr Banks, Hanks plays Walt Disney, the moustachioed showman who turned a cartoon mouse into an animation empire. As the film's director John Lee Hancock says, "It was a list of one - we had to get Tom Hanks to do this because who else can play Walt Disney?" And it's true: Hanks does wholesome like few others (he's tried the opposite, in films like Road To Perdition, with less success). It's not an act, either. He really is this gosh-darned nice.

You might say Hanks, 57, is one of the family having worked on the Toy Story franchise - produced by Pixar, the animation company owned by Disney. He might say he's part of the furniture. "I used to have an office at Disney," he tells me. He even went looking for it during his prep for the movie. "They'd blown out a wall and it was now a conference room for some DVD marketing department," he sighs. "I was looking for the plaque or blue plate - nothing, nothing!"

Oddly, given how their sensibilities align, Hanks has only made one other film for Disney - 1989 cop-canine comedy Turner And Hooch. Perhaps it was because, after charming his way to a first Oscar nod in Big the previous summer, he had loftier aspirations. Within four years, he'd won an Academy Award for playing a victimised Aids sufferer in Philadelphia. A year later, his second Best Actor Oscar arrived for his Zelig-like man-child Forrest Gump.

While he followed this with epic leading man roles in Apollo 13, Saving Private Ryan and Cast Away, as well as making his directorial debut with That Thing You Do!, showboating has never been the secret to his longevity. "He's more than willing to do whatever you want," says Banks's director Hancock. "He wants to be an actor in your movie. He wants to serve the movie through your vision. He's very humble and very professional in that way, willing to try whatever you want."

Capable of serious and sweet-hearted, he channels both in Saving Mr Banks, which deals with Disney's relationship with PL Travers (a brilliantly brittle Emma Thompson), the terribly proper British author of Mary Poppins. For 20 years, Disney tried to persuade her to give him the film rights to her children's classic about a magical nanny, only for her to refuse, afraid that he would sugar-coat her story. Scripted by Kelly Marcel, the film picks up as Travers, desperate for money, finally relents. Their combative relationship, as Travers fights to preserve her creation while Disney tries to kickstart his, is at the heart of Hancock's film.

Impressively, for all its sentiment, it doesn't soft-soap its depiction of Disney. "He was a businessman, at the end of the day," says Hanks.

"For all the whimsy that goes into it, and all the true specialness that he thought was the potential of Mary Poppins, he wanted to make a movie because he had a lot of money sunk into the rights."

Particularly remarkable is the fact that a Disney-produced film shows their creator-in-chief as a man, not a figurehead. In the film, he drinks and smokes. "He liked Scotch. And he did die [in 1966] of lung cancer," notes Hanks. Still, these vices could have been whitewashed, particularly nicotine. Instead, it's drawn attention to. "They always knew he was coming down the hall for meetings because they'd hear him cough," says Hanks, who spends much of the film hacking across his lines.

Playing Disney was "a burden", he admits. "I did not do it lightly." He spent time with daughter Diane Disney, who died earlier this week. Before her death, at the age of 79, she had given Hanks full access to the wealth of material held at The Walt Disney Family Museum in San Francisco. "He was always pumping the money right back in the product, so he was always one pay-roll cheque off something terrible happening," says the actor, admiring his subject's entrepreneurial dice-rolls.

Hanks is unequivocal when I question if he sees anything of Disney in himself. "No. Nothing. Which is part of the pleasure. He's nothing like me." Hanks is not that corporate. His behind-camera ventures are more eclectic. Yes, he executive produced Band Of Brothers and The Pacific, two Second World War ventures, with Steven Spielberg. But look at his other credits and you'll see British University Challenge comedy Starter For 10, mega-hit My Big Fat Greek Wedding and this month's JFK drama Parkland.

Has he ever pursued anything with the relentlessness that Disney chases Travers? "No," he says, shaking his head. "I've had the rights to things and then tried to crack them and get them made, and it hasn't worked out. Often times, there's just something wrong with the material. I've never said 'Oh, there's these rights for something I really want to pursue.' I'm not that kind of businessman... I'm in it for the fun! That's too much work as far as I'm concerned. Who wants to work that hard?"

Raised in California, Hanks remembers Disney as a big part of his life growing up with his two brothers, Larry and Jim, and sister Sandra. "Walt Disney's Wonderful World Of Color was on every Sunday night at 7pm and that was the reason you wanted to have a colour television. To see whatever was on Walt Disney's Wonderful World Of Color. And if a friend in your neighbourhood had a colour TV, you knew where you were going to be on Sunday. You were going to be at his house!"

He can remember his father Amos, a cook, taking the kids to the Golden Gate theatre in San Francisco to see the re-issue of Disney's 1954 Jules Verne adaptation 20,000 Leagues Under The Sea "with Kirk Douglas and the giant squid". And Mary Poppins? "I don't think I saw it in its original release because my parents were busy getting divorces from each other," he says, without a flicker of emotion. "They didn't shunt us off to the happy family films at that time."

In fact, Hanks's folks divorced in 1960, four years before Mary Poppins was released. But you get the point. When they did, Sandra, Larry and Tom went with their father; Jim stayed with their mother, Janet, a hospital worker, in Red Bluff, California. Hanks has seen divorce in his own life too, separating from his first wife, actress Samantha Lewes, mother to his first two children, Colin (now a flourishing actor in his own right) and Elizabeth Ann.

Since 1988, he's been married to actress Rita Wilson, mother to his younger sons, Chester and Truman. Their marriage is famously one of Hollywood's most rock solid; ask him why and he simply shrugs, hinting he knows just how fortunate he is.

"The standard version of a mid-life crisis is that people wake up one morning and realise that they are unhappy, even though they have everything. I don't know what that is. I might wake up tired in the morning but I don't wake up unhappy."

Hanks is not one for drama, despite it being his profession. He recently revealed on the David Letterman show that he has Type 2 diabetes, but refuses to make an issue from it. Likewise, he's not one to let his fame get in the way of a day out. "When you're out in plain sight, with a hat and glasses on, you can get by anywhere." And when I suggest Saving Mr Banks might see him up for the sixth Oscar nomination of his career, he bats that one away too. "What are you going to do? I guess. OK. Fine."

For all his A-List connections, Hanks distances himself from the grimy mechanics of the business. When I ask if he will resume his vocal duties as Woody for the mooted Toy Story 4, he says "I don't know. That is like corporate level." He says the same for Robert Langdon, Dan Brown's professorial creation he played in The Da Vinci Code and Angels And Demons, and whether he will return for The Lost Symbol. Hanks prefers his view from the ground level, with the rest of us.

Saving Mr Banks is in cinemas from Friday