In the 50 years that the writer Jay Cocks has been friends with Martin Scorsese, the two have collaborated on some films that were made quickly – The Age of Innocence took all of 17 days to write, and it went into production almost immediately – and others, such as Gangs of New York, which took decades to bring to the screen. Their latest collaboration is Silence, an adaptation of the novel by Shusaku Endo about two young Portuguese Jesuit priests in the 17th century (Andrew Garfield and Adam Driver) who risk their lives and faith to travel to Japan to investigate rumours that their mentor (Liam Neeson) has committed apostasy. As the film enters the awards-season fray, we spoke with Cocks, 72, about how the film grew out of attacks by the Christian right on The Last Temptation of Christ, on working with Scorsese and on how to maintain enthusiasm for a project that takes almost 30 years to come to fruition.

Let's start with just the facts. How many drafts did you do on Silence?

I think at least a couple dozen.

Have you figured out your hourly wage on this?

I don't get paid by the hour.

But how many hours would you say you worked on it?

Oh, a couple thousand at least. Not counting time seeing rough cuts and talking, stuff like that.

You began working on it in 1989 or so. How long before you had a draft that really worked?

Marty says in 2006 is when he feels we really got a lock on the structure.

What took so long?

Do you know the story of how I came to be involved in this? Marty gets a hold of the book, he reads it as the dust is settling from Last Temptation, he says, does this sound interesting? And without reading it, I say, you bet! Because whenever you throw your lot in with Marty, you know that it's going to be a great adventure, and a great film. So – without reading it, I said yes. And I read it and went: Ho-ly smoke.

Why?

The novel is, shall we say, challenging, for a couple of technical reasons. The translation is very poor, and sometimes confusing. I mean, there seem to be even proofreading problems with it. It's a book told in several voices: Third-person narrative, first-person narrative, epistolary narrative and, at the end, an official document and excerpts from the history of a Dutch trader. So, you have to take all these voices, and the incidents they relate, and put them into some sort of dramatic structure. I embarked on doing that, and the producer at the time called me up and said, You should stop, there are money problems.

Even back then?

Yeah, that's right. This would have been 1992. I said, I can't stop, I gotta do this for Marty. What I didn't say was, I was really lost. I was lost in, as the book says, the swamp of Japan. I was lost thematically, I was lost linguistically, I was lost spiritually, I was just a blind man in a snowstorm. It was the scariest writing experience of my life.

But you gave Scorsese a draft and he liked it.

What he particularly liked was something I did with the ending that is not in the book. So, having done this, and felt rather good about the ending myself, I had to say, well, I guess on some level I did understand this material. Now I have to go back and figure out what it is I understood about it.

So, the two of you then worked on a series of drafts, as legal and money problems came and went. How do you maintain interest in a project that takes this long to come to fruition?

I have a one-word answer for that: Marty. Because his faith never dimmed, his enthusiasm never slackened, it only deepened, along with his insights into this material. Several people have asked him, what would have happened if you'd made this when you guys first wrote it, in the early nineties? And he says, I don't know, it would be a combination of Goodfellas and The Age of Innocence, if you can imagine such a combination. But although it was very frustrating and very enervating to wait this long, I think the fact that he made this at full maturity, as an artist, was invaluable to the movie. So – I didn't appreciate the wait, but I'm glad at the results.

The film is a celebration of faith, but ambivalent about the imperative of spreading religion into foreign lands. And it comes at an interesting cultural moment, when we're newly wary of religious fundamentalism. But you and Scorsese had your own battles with fundamentalism in the late eighties.

I believe that the hullabaloo over Last Temptation may have been the first stirrings of the strength of the Christian right. I think the resistance to that movie from the right wing, and the way that resistance was met – in many cases with just kind of knuckling under – gave the Christian right a sense of its political and cultural power that maybe they didn't have before. And after rallying around that, they went rallying around political candidates and presidents and flexed a lot of muscle.

Given that, are you at all conflicted about celebrating faith?

No! The Christian right is a distortion of faith. It's a distortion of the Gospel of Christ. It has nothing to do with why Christ has been such a seminal cultural figure for centuries, it has nothing to do with love, tolerance, understanding. It has to do with reactionism. It has to do with bigotry, and that's not Christianity.

Silence is called 'A Martin Scorsese film.' But you were the one toiling at the writing table for all of those hours. How does that feel?

I feel proud! The analogy I use sometimes is, on my best days, I'm Nelson Riddle and he's Frank Sinatra. I just arrange the song for him and he just goes and delivers it like nobody else. To have my name on a Martin Scorsese movie? I couldn't be happier, prouder or more fulfilled. And so, if you're a writer, and you think – okay, I'm going to put my brand on this too – you're making a mistake. Because you're not there for the trench warfare that goes on during shooting. You're there during the sunny days just before the war.

This interview has been edited and condensed.