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Finding both a younger Nigel and an older Nigel to represent the two areas of Slater's youth that are covered in the film was not an easy task.

"We were looking for someone who carried the spirit of Nigel," says SJ Clarkson. "It's always difficult casting child actors because we wanted to find someone fresh who hadn't done much before. Our brilliant casting director Rachel Freck and I both wanted to find a boy who captured the spirit of Nigel, who had that naivety and innocence."

Oscar Kennedy, a young aspiring actor from Nottinghamshire, came to the audition with very little acting experience. He had been training at the Nottingham Workshop, a forum for young actors, when he received a call from the production to come to auditions.

"Oscar has a rare and wonderful quality of innocence and wisdom running parallel, which was perfect because I think that's who Nigel was. Oscar had a great understanding of what was going on around him but yet he was this little lost boy," Clarkson remembers.

"He was initially quite quiet – he just sat there. He had long and cool, skater-boy hair and we asked him to clip it back, which he hated. I think we brought him back into the room 10 times that day to work with different children and to see how he compared with the others. It's such a big ask for a child who's never really done anything like that before but I believed in him enough to take that risk. He still has the innocence but is also wise with it. I could talk to him like a grown up."

Freddie Highmore is no stranger to working as a young actor in major films. The star of Tim Burton's Charlie And The Chocolate Factory, Luc Besson's Arthur and the Incredibles and Marc Forster's Finding Neverland, Highmore is sensitive to the requirements needed to work among a busy crew and deliver on request any emotions needed to convey Nigel's journey from young boy to young man.

"Freddie is just fantastic," says Clarkson smiling, confident in the knowledge that he has been able to fill Nigel's shoes. "I don't think I've worked with a more technically brilliant actor. I remember there were a few scenes with a lemon meringue pie where he would have to slightly waft it past camera or Mrs Potter and lift it up and bring it perfectly into frame and he just could do it in one take. It was extraordinary. As an actor, I think he's always flawless in whatever he does. To be able to make those looks to camera and get them just right is hard. I think we really lucked out with Freddie."

"Thank goodness he said 'yes' to playing the older Nigel," says producer Faye Ward. "It was so hard to find a match – to find two boys who are both great actors and who encapsulate the real person. When we met Freddie he had quite long hair, actually quite like Nigel's now, which we completely chopped off to match the Nigel of 1970. But he also brought a physicality that really lent itself to Nigel's character. Freddie is a true film star, he offers those extra unrehearsed moments that give true life to a character and light up a scene."

Casting Mrs Potter was another hurdle because, for the team, her presence had to bring conflict without it resorting to a two-dimensional clichΓ©, and also bring pathos, because Potter was a real woman being portrayed in what is ostensibly a biographical account. "I think she's a wonderful character," says Clarkson.

"Toast is very much played from a little boy's perspective so could be seen as being slightly warped, and we were very conscious to always make sure Mrs Potter didn't come across as two-dimensionally horrid, because actually, she was a real person and thereβ€˜s always more than one perspective to a story and I felt we should show that, and that we should humanise her."

Clarkson had immediately pictured Helena Bonham Carter in her mind as Joan Potter, the home help who won Nigel's father's heart and ultimately became Slater's nemesis. But Bonham Carter was stepping from one major Hollywood picture to the next with barely a moment's pause.

"When we first started talking about how brilliant it would be to have Helena play the part, there were a few raised eyebrows because we're talking about a Midlands working class woman, which is quite a long way from who Helena is. But I have seen her do so many roles in the past where she's just absolutely grabbed them all and taken them to the extreme. Casting her was one of those rare and wonderful moments where you have your dream actor in your head and they actually say yes."

Slater admits he had painted Mrs Potter as a nasty woman in the book: "I gave Mrs Potter quite a tough time because she gave me a tough time," he says. "The first meeting I had with Helena, she said something that really took me by surprise: 'What about the tender moments? You know it can't have all been bad.' And it did slightly hit me like a brick really because, of course, there were tender moments."

"She taught me how to tie up my football boots; she taught me how to tie a tie; she showed me how to knit. And those are quite intimate moments. We did laugh about things as well, about my dad sometimes. They didn't feature in the book and they certainly didn't feature in my memory of Mrs Potter.

"It was only when Helena said: 'Oh, come on. What about the other side?' that I began to explore that side of things. So Helena's portrayal, for me, has actually woken up a side of me that was well and truly sleeping. Not that I'm suddenly fond of Mrs Potter's memory. I haven't fallen in love with her. I was just unaware really that there was a hidden side. I'd forgotten about it.

"I feel a bit warmer now to Mrs Potter than I did. She doesn't send scary shivers down my spine any more, which she used to. So maybe there are several good things that have come out of this."

On the casting of dad, Clarkson agrees that it was difficult to find someone who could convey the meanness of Slater's memoir with the vulnerability the writer conveys with his affection for the man.

"I think Ken showed that brilliantly," says Clarkson. "They were living in a time when you didn't show your emotions and your feelings; you didn't cry. There are some wonderful moments where Ken has humanised dad. There are moments that I find heart-breaking. To pull off a character that could seem one-dimensionally horrid throughout, and to find all the other facets of that character, was very clever.

"Ken was one of the people I didn't talk to before we filmed," says Slater. "There was a very good reason for that. The minute his name came up, something inside me just relaxed. I just thought: 'Of course – that's dad. Ken Stott is my dad.'

"When I was having a chat with Freddie in the make-up room, I was just thumbing through the book of the photographs – how everybody had been made up so far – and I turned a page and this chill went straight down my spine. I was looking at my father's neck, his haircut. It was just like I was looking at the back of my dad and I turned the page and it was Ken.

"Every movement, every detail, every word is my dad and it was difficult because I had very tough times with my dad and I haven't got many good memories of him. But I knew that the character was in perfect hands and, you know: 'Sorry, Ken, you're just like my dad!'".