The Sunday Times Chief Sports writer David Walsh spent 13 years battling to expose Lance Armstrong - one of the most celebrated men in sporting history - as a cheat. His book Seven Deadly Sins has been adapted into a movie The Program which hits UK cinemas this week. To coincide with the film's release David has written a blog describing what happened when Hollywood came knocking.

From the beginning, everyone knew there would be a movie on Lance Armstrong. Bud Greenspan was going to make it and it would glorify the career and life of Lance. As the years passed and questions were raised about how Armstrong had become so good, Bud's enthusiasm cooled. He passed away in 2010 but still the talk continued - Matt Damon would play Lance, no Bradley Cooper was already signed up. So it went.

Back in those days, the most I could have hoped for was a bit part as a minor villain. A small-time Joker to Lance's Batman. But I never truly thought there would be a movie that would reflect the part of the sceptical, disbelieving journalist. I had been a thorn in Lance's side and it was always clear that when he was being celebrated by some fawning chat show host, he steered away from speaking of those who didn't believe in him, trolls like me. When the time came for Hollywood to embrace in, the love-in wasn't going to give much credence to "the cynics and the zealots, those who don't believe in miracles, who can't dream big."

That's how Lance referred to us in 2005, at the moment of his seventh consecutive triumph in the Tour de France. From the Champs Elysees that Sunday evening, he rode off into the sunset, heading towards Sunset Boulevard no doubt. But the story would change. Lance would be outed as a cheat, and from being "the little f*****g troll" I became the "dogged reporter who had pursued the story for 13 years." But I hadn't changed, just the story.

And the narrative for the movie changed too. The hero became the villain, and the troll morphed into an investigative journalist on the trail of the truth. I was involved in The Program from the beginning and the film tells the story of what it was like the journalist who most people viewed as some kind of pariah. To have Chris O'Dowd play my part wasn't bad either, he's over six feet tall and I'm just a small guy. So that's been a dream. But I was involved from the start, consulted on every draft of the script, shown every version of the film.

What do you think, what do you think, they asked time and again.

I thought, 'I think this is all just brilliant,' but I tried to sound a bit more cool than that.