Ol Parker's 2010 film, The Best Exotic Marigold Hotel, about a group of British oldies finding love in a crumbling retirement home in India, gained unexpected success as a previously untapped audience - the over-60s - came out in their droves to watch it. Several film studios, such as Working Title Films, decided to pass on the pitch, considering it unmarketable, but, Parker says, his latest film was even harder to sell to film executives.

Now is Good, out this week, is based on the novel Before I Die, by actress-turned-author Jenny Downham, about a young girl living with a rare form of leukemia that has progressed so far that she is given a terminal diagnosis. It isn't the sort of topic that can produce a Hollywood-style happy ending, but Parker was determined to get it made.

"I read the book in one sitting and I woke Thandie [Newton, his wife] up that night because I was crying so much. From then on, it was something I was always desperate to make into a film. Whether I would ever get enough support to make it was another matter."

One element that unites the two films is they both deal with characters who know that death is a certainty, and are trying to live life to the full. Parker admits he was contemplating his own mortality when he chose them. "Before you turn 40 you think, "everyone else is going to die, but not me", then you realise death is certain for everyone. I was on the cusp of turning 40 as I was writing them, so the theme of ‘however long you've got make the most of it, and have an amazing time' really resonated with me."

The main character, Tessa, is trying to live life to the full while she still can. She makes a bucket list of goals including "have sex" "take drugs" and "break the law". Played by Dakota Fanning, Tessa is very difficult and hard to sympathise with initially, but after she falls in love with the boy next door, Adam (played by Jeremy Irvine) a softer side to her character emerges.

"Being a teenager anyway is incredibly intense and every moment is invested with ferocious importance. So many firsts happen around that time - imagine your first kiss, or fag, or shag and then layer impending death on top of that. We tries to bring out that tumultuous emotion in the film, and that's why she has that brackishness. I liked that she was tricky and bulshy and stroppy. That seems very human. Dakoto plays it so well, she is just so ill all the time, you can see it in her posture, her slightly hunched shoulders. Then, after she finds love, she changes and becomes alive again, I remember that night when we filmed the beach scene [where Fanning and Irvine run into the sea at night] and her eyes lit up, it was just beautiful."

The decision to make the Brit flick was partly born from Parker's desire to explore his own grief and recovery after his friend died.  "It wasn't cancer, he drowned actually. I knew I wanted to write about grief after that. It wasn't the cancer or even the death in the book that attracted me. It was the life." The film's scenes, though desperately upsetting at points, manage to end on a life-affirming note. "I wouldn't make the film that was an hour and half progression from sadness to despair" he laughs, "I'm hugely optimistic".

Although the focus is on the life, Parker also explored the disease (acute lymphoblastic leukemia), with a doctor on set to make sure any medical details were accurate. He also met  Alice Pyne, a 16-year-old girl who was diagnosed with Hodgkin's lymphoma four years ago. Her blog gained international attention after she published a bucket list, and she has campaigned to get more people on the bone marrow register. The two are now friends and Alice attended a screening of the film.

"It was amazing to have her sitting in the audience. She and her family were there and they are a wonderful family, they laugh a lot and are really funny and warm. There is a scene in the film where Cal [Tessa's little brother] says something wildly inappropriate, like "when Tessa dies can we all go on holiday to Spain?" and they all burst out laughing because that was exactly how Alice's younger sister Millie was when Alice was diagnosed.

There's a rustling of paper, and then Parker reads from a letter sent from Alice's mother after the screening: ""We laugh all way through our life, even during the bad bits, so we can relate," she says. I have learnt a lot from them. Alice's mum is climbing Mount Kilimanjaro at the moment. She planned to do it after Alice died, and scatter her ashes at the top.  But Alice is still here, so now she's going to climb it twice."

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