Esio Trot: Judi Dench and Dustin Hoffman are a perfect match in the BBC's Christmas special

The two Oscar-winning actors in the festive adaptation of Roald Dahl's children's novel about a pair who bond over a tortoise tell James Rampton how they bonded without the tortoise too

Flower power: Dustin Hoffman and Judi Dench in 'Esio Trot' (Endor/BBC/Nick Bridge)

The Independent walks into a bar – and it is like entering a 1970s time warp. At this smoky nightclub in north London, the stage is fringed with gaudy red and gold strips of tinsel, the walls are decorated in brown wallpaper which probably was not even fashionable in the decade that taste forgot, while the floor is covered with a red, patterned carpet that sticks to your shoes with every step. On a side table sit prizes for a raffle, including a bottle of bubble bath, a pair of carving knives and some elderflower cordial.

It is not a location dripping with Hollywood glamour, nor a place where you would expect to encounter two Oscar-winning actors widely regarded as among the finest of their generation. But here we are, watching Dame Judi Dench and Dustin Hoffman, no less, perform a pivotal scene from Esio Trot, Richard Curtis and Paul Mayhew-Archer's charming adaptation of Roald Dahl's best-selling children's novel, which will be shown on BBC1 over Christmas.

In this one-off film, which is narrated by a chirpy James Corden, Hoffman plays the tongue-tied bachelor Mr Hoppy. He is smitten by his flamboyant downstairs neighbour, Mrs Silver (Dench), but is far too shy to do anything about it. She in turn is too preoccupied by the fact that her beloved pet tortoise, Alfie, is failing to put on weight to notice that Mr Hoppy is suffering from a bad dose of unrequited love.

So he tries to make her happy by convincing her to whisper to Alfie thrice a day a magic spell – "Esio Trot" ("tortoise" backwards) – that will make the creature grow. When she is out, Mr Hoppy clandestinely replaces Alfie with a succession of ever larger tortoises. But will love blossom over the tortoiseshell?

Dench and Hoffman have an obvious rapport on set. In between takes – Mrs Silver is participating in a dance marathon dressed as a flapper girl, while the lovelorn Mr Hoppy, wearing his regulation-issue, beige tank top, looks on longingly from the sidelines – they josh and finish each other's sentences in a way that only really good friends can.

After Dench has been dancing for a good while, for instance, Hoffman jokingly asks his 80-year-old co-star: "Judi, do you need a nap?" Later, he exclaims that they are working so well together on Esio Trot, they should collaborate again immediately. "Let's do Who's Afraid of Virginia Woolf? next. After this, we'll do that play and beat the crap out of each other!" They both howl with laughter.

It is quite a coup for BBC1 to have landed two stars of this magnitude, as an anecdote from Curtis illustrates. "My favourite moment almost of the whole shoot is when James Corden finished the film. I overheard him saying goodbye to Dustin Hoffman. And he said, 'Dustin, it's been a lovely experience. I'm going to spread the word. You're a smashing little actor.' So maybe Dustin will pick up some other jobs now."

The chemistry between Dench and Hoffman – both on and off screen – is plain to see. They ended a recent joint appearance in London by dancing on stage to Louis Armstrong's "A Kiss to Build a Dream On" – just as their characters do in the film. The performers also share a wicked sense of humour; at one point, Hoffman jokes that he preferred working with the 60 on-set tortoises than with Dench.

After filming has finished, the pair discuss what drew them both to Esio Trot (and no, it wasn't a shared love of tortoises). Dench, who has enjoyed a stellar career on stage and screen since making her professional debut as Ophelia at the Royal Court Theatre, Liverpool, in September 1957, says the idea of working with Hoffman was all it took to convince her to accept the part of Mrs Silver. "I knew the story. I've read it to children. Many, many children.

"And, well, they did say Dustin Hoffman's name. So, I mean, it could have been Five on a Treasure Island or whatever. It could have been anything. It could have been just, 'Would you like to come and walk down the street with Dustin, and Richard Curtis will watch you?' I wanted to play Mrs Silver, unconditionally." She adds that "What I liked about it was boasting that I was going to do it, before we started. And now I say, 'Oh, I know Dustin.'"

For his part, the 77-year-old Hoffman, who has starred in such films as The Graduate, Midnight Cowboy, All the President's Men, Marathon Man, and Tootsie, was just as eager to work with Dench. "Do you know, I saw Judi in Mrs Brown in the States? I was so taken with the performance – and I rarely do this – I said, 'Is it possible to get her phone number?' I got her phone number and I called her.

"I started going on and on and on about how brilliant I thought she was in the film. And she kept trying to interrupt me. And I kept going past the interruption. And finally she said, 'I really have to be on stage now.' It was between the first and second act that I'd called her mobile phone. She was on in the West End. So that was our first encounter."

The actor continues that his admiration for his co-star only increased when he saw online pictures of the actress as a younger woman. "You just push a button, and there's Judi in her teenage years or her twenties. Equally gorgeous now, but I mean stunning then. And I said to Judi, 'If I'd met you then, I wouldn't have let you get away.'

"There must be a way to do a love story where we meet in our twenties, yet we're acting as we are now – with computer graphics or something. Why can't we look like we did in our twenties? If only we could do it. 'If Only'. There's your title."

Hoffman, who won Best Actor Oscars for Kramer Vs Kramer and Rain Man, goes on to elaborate on what he thinks makes his co-star such a special actress. "What makes Judi so extraordinary, I think, more extraordinary than other extraordinary actors, is this blend of character and herself, so that the character never runs away from the actor. She blends herself. She's in there, every molecule of herself.

"So when you're watching this flamboyant Mrs Silver, and there she is showing Alfie these photographs, and she starts talking about her husband, Judi comes through there. It's chilling. And, for my two bucks, you won't get better acting than that. Where you see this blend, it just gets you. She just allows you right into her bone marrow, as it were."

Mrs Silver certainly seems to share some of Dench's mischievous qualities. At one point in Esio Trot, the character puts up a Christmas tree in her front room in August and says, "When you get older, if you can't break the rules, what's the point?"

The actress, who scooped an Oscar for Shakespeare in Love in 1998, observes: "I think it's quite fun to break the rules at any time. I don't think it necessarily matters that you're getting that dreaded word, 'O-L-D-E-R'. We've always been like that from the beginning. So I think it's good not to conform, actually. If you don't want to toe the line and you want to break the rules, go ahead."

Dench adds that putting on Mrs Silver's very showy outfits and eye-catching red wig helped her inhabit the character. "I loved all that. She looks very unlike me, which is heaven. It's not so much fun looking like yourself! And in that red wig and all this costume, I did feel like Mrs Silver, not like me. What a relief.

"Afterwards, I was offered one of her dresses – a white, flowery dress. And the costume department said, 'Would you ever wear it?' I said, 'Of course I'll wear it.' Well, of course, it hangs in my cupboard, and I look at it, and I think, 'When am I ever going to wear that?' Well, not looking like this!"

Hoffman picks up the baton, explaining how he got into his character. "In this business, if you become successful it's a freak accident, I always think. The longer we live, the more we know so many talented actors who just weren't at that place at that time and just wind up much less fortunate than we are. So I start in this part thinking, 'Well, what if that didn't happen to me?' I could see myself like that.

"I guess that's what it comes down to: can you see yourself just living alone? And I could see myself. I mean, this has all been a dream anyway, right? I think I'm going to wake up with tubes coming out, and I'll say, 'You mean I really didn't become a star? I've really been unconscious for 50 years?'"

Perhaps what keeps their performances so fresh is that Dench and Hoffman are clearly so open to taking on new challenges. Dench reflects: "I think you learn from every single thing you do. I've always liked doing the most different thing from the last thing I've done.

"Now the last thing I would want to play is anybody like Mrs Silver. The last thing. You play one thing, and then suddenly getting a part like Barbara Covett in Notes on a Scandal, an absolute gift. You think, 'Oh, another kind of stimulus. Something else to get hold of. Some other person to find out about and try and portray.'"

In the same way, Dench says, "I loved playing Cleopatra because people were openly rude about me playing the part. So the challenge of that was absolutely tremendous."

According to the actress, "I've never done a part where I haven't learned something new in it. I remember Michael Williams, my husband, and I did Diary of a Nobody in the theatre. We said, 'This is very short. We'll just do this. Then we'll go home, and it'll be absolutely wonderful.'

"Well, it was one of the hardest things I've ever, ever, ever done. So things always present a challenge, I think. Always. And the more challenge it presents, the better you feel. And the more miserable you are!"

In the end, Esio Trot is a beautiful love story and a timely reminder that it's never too late. Its two stars are life-enhancing proof that age is not a barrier to love or to producing wonderful, rich work.

Hoffman concludes: "There's a line from Bertrand Russell that I have written down on my bulletin board. When he turned 90, they asked him, 'How does it feel to be 90?'

"And he said, 'Oh, to be 80 again!'"

'Esio Trot' is on 1 January at 6.30pm on BBC1