Everyone was taken by surprise last June when Daniel Day-Lewis, the undisputed greatest actor of our time, announced his retirement. Now, at last, we have some kind of explanation as the thespian has spoken about what led to this drastic decision for the first time.

The Wicklow-based actor opened up in an interview with W magazine, explaining that the decision came upon him after filming his upcoming much anticipated final movie, Phantom Thread, by director Paul Anderson.

"Before making the film, I didn’t know I was going to stop acting. I do know that Paul and I laughed a lot before we made the movie. And then we stopped laughing because we were both overwhelmed by a sense of sadness.

"That took us by surprise: We didn’t realize what we had given birth to. It was hard to live with. And still is."

The 60-year-old actor, who is the only performer to have won three best actor Academy Awards, is known for his method approach to acting and will completely engulf himself in a character. In preparation for his role as couturier ­Reynolds Woodcock in Phantom Thread, Day-Lewis re-created a Balenciaga dress, not an easy feat by any stretch of the imagination. But then this is the same actor who didn't break character throughout the filming of My Left Foot, and learned to throw knives with pinpoint accuracy for his role as Bill "the Butcher" Cutting in Martin Scorsese’s Gangs of New York.

Despite the varied roles he has played over the years however, it was the intensive process involved with making Phantom Thread and his subsequent decision not to watch the film that is part of his reason for quitting acting.

"I haven’t figured it out," he said. "But it’s settled on me, and it’s just there. Not wanting to see the film is connected to the decision I’ve made to stop working as an actor. But it’s not why the sadness came to stay. That happened during the telling of the story, and I don’t really know why.

"I dread to use the overused word 'artist', but there’s something of the responsibility of the artist that hung over me," he added.

"I need to believe in the value of what I’m doing. The work can seem vital. Irresistible, even. And if an audience believes it, that should be good enough for me. But, lately, it isn’t."

Daniel with his wife, filmmaker Rebecca Miller.

The actor felt it was important to make an official announcement about his decision, saying, "I knew it was uncharacteristic to put out a statement. But I did want to draw a line. I didn’t want to get sucked back into another project. All my life, I’ve mouthed off about how I should stop acting, and I don’t know why it was different this time, but the impulse to quit took root in me, and that became a compulsion. It was something I had to do."

It's still a choice he is coming to terms with himself saying, "Do I feel better? Not yet. I have great sadness. And that’s the right way to feel. How strange would it be if this was just a gleeful step into a brand-new life. I’ve been interested in acting since I was 12 years old, and back then, everything other than the theater—that box of light—was cast in shadow. When I began, it was a question of salvation. Now, I want to explore the world in a different way."

Phantom Thread will be out in cinemas in February 2018, and if you're as curious as we are about this movie, you can watch the trailer below.