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"If there's one scene that sums up Mrs Potter, it's the one when Nigel comes home with the scones," says director SJ Clarkson of the character that seems to haunt the entire project. "Cooking is Mrs Potter's arena, so when Nigel comes in with the scones, her territory is threatened. She literally doesn't know what to do with herself and it's as much as she can do to get out those lines 'Well done, son.' You can see how angry she is, suppressed anger at scones being brought to the table, but, equally, her being threatened by Nigel and realising she's got to up her game. So she brings out her tour de force lemon meringue pie."

Helena Bonham Carter laughs when asked what her portrayal of Mrs Potter brings to the project. "Well, Nigel's put his childhood to bed. You know, I think it has been a therapeutic for him... All down to me!" Bonham Carter says this with a beautifully honed Wolverhampton accent that she maintained both on and off set for weeks. She commented that it began to drive her family to distraction as she continued to use it at home.

"It's all these different gradations. And then as soon as you write something which is dramatic, you're going to change people," she says. "And you serve the story. You don't serve the reality. But Nigel seems happy and at least I might have helped him get over a few childhood traumas. If I have, then, you know, then at least I've done something good."

Bonham Carter is one of the most sought-after actors of our generation, having starred in more than 40 major films in her career to date. She has won numerous awards and has played a number of controversial and challenging roles throughout her career. Director SJ Clarkson was amazed that she agreed to take this role, as it was such a shift from her recent portfolio of work on the big screen.

When asked whether her priorities have changed she says: "Yes, completely. As soon as you have children, criteria change. Obviously, if the writing's not good, you're not going want to do it. I've always been intrigued by Nigel and I love his cookbooks. When you read Toast, it's a great way of just getting catapulted back to your own childhood. And also getting inside a boy's mind... it was helpful for me to get in touch, in a way, with my own son. And how much a child is so particular about food."

"But then there's the logistical thing. I can't go away – you know, during school term time. It was a three-week shoot. It was in the summer holidays. It meant that I could do a five-day week. They could accommodate that. I could be back for weekends."

How was she convinced that this was a part she wanted to play? "I met SJ and she talked about her vision of the film, which was very precise," Bonham Carter recalls. "She talked about her 'Amélie-esque' vision and I really loved Jean Pierre Jeunet's Amélie. So we just got on. As she had such a clear vision, I thought: 'She's not going to let me down here.'"

After discussing the role in great detail, Lee Hall was brought in to do some rewrites across Mrs Potter's character, to bring as much colour and contrast to her as possible to enable the audience to see her as a human.

Bonham Carter's relationship with Slater soon grew and she used his knowledge and memory to research elements of her character. "We started a textual relationship," she laughs. "He'd just text me telling me about my character – 'What magazine did she read?' 'She magazine.' All the information that he remembered, real precise details: 'Cigarettes?' ‘Woodbines Number 8.' I think Nigel did revise his opinion of her during the whole process."

"When you actually see that cigarette being lit and it's the same cigarette that she would have smoked, and the sort of lighter my stepmother would have used, even the way she blew the smoke out... Suddenly you realise that a short sentence – she lit a cigarette – can become such a big thing on screen," Slater explains.

Bonham Carter in a way finds it difficult not to defend the woman she has been given the responsibility of portraying, warts and all. She notes that, although she would never allow Nigel to encroach on her cooking, there were many other areas of domestic life that she introduced to Nigel, including sewing and knitting.

"The father wanted this butch boy, you know, who could play football, but Nigel just wasn't interested," she says. "I think he always felt that he let down his dad. Mrs Potter, meanwhile, had no trouble with him liking the domestic side – except when it came to cooking, which I always found was hilarious. She's as much a child as he is and she's so insecure that she doesn't want to be upstaged by this little boy."

Bonham Carter spent many of her scenes opposite Ken Stott who plays Nigel's dad. Stott, alongside Bonham Carter, has starred in a library of major motion pictures, including Charlie Wilson's War, Shallow Grave and ITV's Rebus. She jokes about the actor's ability to effortlessly deliver an extraordinary performance as Nigel's troubled father.

"He does nothing and yet he's just brilliant. Barely read the book. I don't think he even read the book. He just piles in, learns his lines at the last minute and then suddenly he's brilliant... but then that's Ken Stott for you!"

"My mum, who's really bright – a psychotherapist – she said, 'Mrs Potter's not the problem,' when she read the script. 'It's the father.' I think Ken brought that out. He's a man who just did not know how to cope with the death of his wife and was knocked for six. He didn't know how to be a father, really."

Bonham Carter is happy with the ending, even though Potter is left alone. "For Nigel: absolute paradise," she says with conviction. "He survived. He got out. And thankfully, he's done so well."