THERE’S a box in David Suchet’s house in London that contains little pieces of who he used to be. I ask him what’s in there. A handkerchief, he says, and a prayer book, and a rosary. There’s also a small ashtray and a toast rack and an egg timer and lots of other little objects and trinkets. Sometimes he thinks it might be time to give them away, or sell them for a good cause, and one day, he says, he probably will. But not yet. Not yet.

He treasures the trinkets because of the man they used to belong to: Hercule Poirot, the detective Suchet played on television for 25 years. When the great actor finally relinquished the role after filming every single story that Agatha Christie wrote, the television company gave him the mementos of his time as Poirot and he says he likes to keep them close by. He also has Poirot’s cane. “Every time I hold that cane,” he says, “I can feel him.”

The cane is particularly special, says Suchet, because of the powers it had when he was playing Poirot; it worked like a kind of talisman, a way for him to transform himself from British actor to Belgian detective. Just before he started filming, he would get the cane out and walk around the garden like Poirot, quick, little steps. He would also speak out loud like him and try to look at the world through his eyes, a man who liked order, symmetry and detail. If the pictures on the walls of his house weren’t straight, he would straighten them. If the eggs he had for breakfast weren’t precisely the same size, he would disapprove. And once in character, he would keep it up and maintain the accent and performance even when he was speaking to his wife.

To people who aren’t actors (and maybe even to some people who are) this might sound a bit weird, but Suchet was utterly committed and methodical when it came to playing Poirot. When he first got the part, he went through the novels and listed every single detail of Poirot’s habits and gestures. He also made a list of ways in which he was similar to, and different from, the detective and the similarities were striking. Occasionally in real life – sans moustache, sans Belgian accent, sans everything – Suchet’s words and phrases can even sound like Poirot’s.

Take, for example, the moment we get talking about nostalgia and regrets. “We do not know the webs we spin,” he says, “and it’s only when you look back that you say, look at how that led to that’.”

It’s the kind of thing Poirot might say while summing up the facts at the end of one of his cases, but it’s also how Suchet likes to look at his life. We humans don’t know what we’re spinning as we go through life, he says, but then we turn around and see the shape of what we’ve been building.

And so, over coffee on the bank of the Thames, we follow some of the threads of silk back through Suchet’s life and find the man who taught him how to feel emotions and the father he was never close to. We also find his faith in God and his fear of death and his feeling, always, of being an outsider and of being on the edge. Poirot also fades in and out as we talk, pointing out, with his usual knack for being right, how alike he is to man who played him.

But first, Suchet, 73, wants to do something. He picks up the camera he carries with him almost everywhere. We’re sitting right by Tower Bridge but Suchet isn’t interested in such London clichés. He’s more interested in the salt cellar and glass on the table in front of us. He points the camera. His forehead tightens, his forefinger slides on the focus. There. Done. “I just wanted to capture the light from outside on the glass,” he says. “What nature had done to it. Oh, look. The moment’s gone. It’s stopped. It’s gone.”

I ask where he gets this passion for photography, which forms the basis of his new book Behind the Lens, and he tells me about one of the most important threads of silk in his life: his grandfather, Jimmy Jarche. Jarche was a newspaper photographer and Suchet particularly remembers the smell of the Syrian pipe tobacco he used to smoke. He also remembers the way Jimmy behaved with him as a boy: he held him a lot, Suchet’s arms wound round his grandfather’s neck. Suchet could also talk to Jimmy in a way that he never could to his father.

“My father was a man of his time,” says Suchet, who has two adult children with his wife Shelia. “He was of his type. He was South African and arrived in this country as an outsider and became an insider because he became this brilliant surgeon. But he developed an Edwardian attitude and part of his personality was that he couldn’t open up. He was closed emotionally, quite unlike me.” Suchet says he finds it easy to love; he enjoys loving.

The relationship with his father was always a problem though – in fact, Suchet says they never really had what you could call a father/son relationship; it also meant Suchet’s father struggled to express his pride in his son when he started to have success as an actor. “He would show off photographs of his children in his office and say how proud he was, but never to us. But I was lucky to have Jimmy who allowed me to have an emotional relationship with an older man who guided me in my life.”

He tells me a story to show what he means. If his grandfather saw a beggar on the street, he says, he would sit down next to him and have a chat and ask if there was anything he could do and Suchet says he still feels the influence of that attitude today. As we talk, people approach the table and ask for selfies and autographs and Suchet is friendly and smiling, always.

“Jimmy is why I don’t hold myself above anybody,” he says. “If I’m well known and famous, that’s happened to me but I don’t see myself as anybody different from anybody else. We’re all travellers; we’re on the same escalator and we’re all going to drop off at the top. If we’re lucky, of course. Some people drop off half way through.”

Which leads to the other great influence Jimmy has had on Suchet. He tells me that, after his grandfather died, he carried on speaking to Jimmy in his head. And then something weird happened. Suchet was lying in the bath in America one day, in the 1980s, thinking about Jimmy when a thought struck him: Why am I thinking like this, when I don’t even believe in an afterlife? Jimmy’s not there. Which got him thinking about the afterlife a bit more. He bought a Bible and read Paul’s letter to the Christians in Rome and began the process that led to him finding his faith.

It took a long time – around 20 years in fact, which reflects one of the similarities between Suchet and Poirot: their methodical approach to work and life. After that moment in America, Suchet researched religion and read and read and read until he could get to what he calls a point of hope and mystery.

“I can’t have blind faith,” he says. “But I’m comfortable to live within a mystery. We are only comfortable with a calculator. We need clear answers. I can’t give you a clear answer; I can’t even persuade you. I wouldn’t want to persuade you.” Interestingly, Suchet says his grandfather was an atheist. “But in a sense he was the epitome of faith. Loving and caring about other people and giving and being generous with himself.”

Suchet recognises that this belief and his faith make him a little different in a country that’s more likely to believe in Richard Dawkins' The God Delusion than The Bible, but he has become used to the status of outsider, in his own country and even in his own profession. He is not a great fan of theatricality, he says, nor is he a great socialite and doesn’t much like theatrical parties. “I’m always on the edge,” he says. “Because there’s a lot of me that’s quite introvert. I don’t enjoy that side of my profession. I feel awkward in it.”

Suchet’s heritage has also made him feel different in other ways. “I don’t know why, but I relate to the outsider, maybe because my blood line is Lithuanian Jew, brought up third generation British,” he says. It’s also made him aware of how Britain, and London in particular, welcomes, or doesn’t welcome, incomers.

“I love London,” he says, “but I can’t bear hearing people say, ‘oh, it’s so full of foreigners’. Do you know why it’s so full of foreigners? Because everybody wants to be here and it’s a funny thing: everybody that comes here become Londoners, they fall in love with it. We welcome people.” It’s the only moment in our conversation where Suchet gets angry.

But here, too, there are similarities with Poirot – Christie’s great detective was an immigrant, a Belgian living in London and sometimes coming up against thoughtlessness and racism. What are the other parallels? Both men are fastidious – Suchet says he would sometimes drive the costume department barmy with his attention to detail. Both men also despise chaos – before he starts a job, Suchet has to start with a completely cleared desk. “I will feel chaos deep within me,” he says. “I have to get rid of it, because it affects me. I have to feel calm and ordered, deep inside.”

We shouldn’t exaggerate the similarities of course, but Suchet does talk about an incident that seems to emphasise them – for viewers and Suchet himself. It happened when one of the Poirot films was being made in Hastings. There was a break in filming and Suchet had gone for a walk. He was in full costume and standing in an ordinary street. Suddenly a little old lady came up the hill towards him. She stopped. “Ooohh. Ohhhh. Hello, Mr Poirot,” she said. Suchet didn’t know quite what to do so said “Hello, madame” as Poirot. ‘Oooohh, ooohh,’ she went. ‘I watch you all the time, Mr Poirot. What you doing here, though? There ain’t been a murder, has there?’

Suchet replied, ‘No, madame. There has been no murder, not here in Hastings.’ ‘What are you doing here then?’ she asked. ‘I am on holiday,’ I replied, ‘I am en vacances.’ ‘Oh, that’s nice. Lovely.” And off she went.

It’s a moving little story and revealing about Suchet. He says he didn’t want to disillusion the old lady but he was also deep in character when she turned up; right in the middle of the serious business of being the Poirot that Agatha Christie created. “That’s my reason for being an actor,” he says. “To serve the writer.”

It can make it hard to move on to the next stage though, particularly with Poirot. “I got to know Poirot after 73 stories and 25 years and I’ve been in nearly every situation with him in life and I know how he reacts and I’ve reacted with him because he’s me. So I know him as well as I would know any best friend. And he dies. He’s gone. I’m grieving. I’ve lost a friend. I can bring him back but only in my memory, like Jimmy.”

Which brings us, via the web of life, to the other metaphor that Suchet used earlier: the escalator, the one that we’re all and the one that eventually ends. Suchet tells me that he likes to go on retreat to find peace and quiet, but he says there are still little and big fears and concerns that bug him. He worries, for example, about what he calls the dangerous society we live in – dangerous because more and more people seem obsessed with fame. You have to be careful not to live your life for other people’s praise, he says.

But Suchet also worries about more personal things. In the old days, he says, if there was something on his mind, his grandfather Jimmy would tell him to come and sit with him. Often, Suchet wouldn’t know what to say but Jimmy would tell him: it doesn’t matter, just sit with me. These days, Suchet has his family and his faith.

So how does he feel about the escalator of life? “I’m frightened of death,” he says, “and nobody has come back since Christ to tell me what’s beyond. You get people who say, my faith makes me convinced and I say to them, you’re so lucky.” But he tries to rationalise the fear. We’re all on the same escalator, he says, sweeping his arm round the room. “Just accept it,” he says, “Don’t worry about worrying.”

Behind the Lens: My Life by David Suchet is published by Constable at £25. David Suchet is raising awareness for the UK charity Tuberous Sclerosis Association which funds research into Tuberous Sclerosis Complex, a rare genetic condition that affects around 1 million people worldwide. For more information visit www.tuberous-sclerosis.org.