One day in 2010 I was walking down Maiden Lane, where JMW Turner was born, and I went into this pub [the Porterhouse] for a quick pint, and I saw Turner’s name on a plaque. So I phoned Mike Leigh because about seven years before he’d mentioned that he was thinking about doing something about Turner, and thought I might be right for it. So I left a message saying I was sitting right under where he was born. I  was about to go home when I got a call asking me to come to his office, where he said: ‘Untitled 2013 is going to be the film about Turner I mentioned. I want you to play him, and I want you to start painting.’ 

You always spend a lot of time working on a character, but preparing for Mr Turner is the most I’ve ever done. They found me a wonderful teacher, Tim Wright, who was absolutely brilliant. His portrait of me is in the BP Portrait Award exhibition [at the National Portrait Gallery until 21 September]. I had two years of lessons. We did life drawing, still life, speed drawing, working in ink, watercolour and then oils. I did about 300 images in all, including maybe 10 quite large watercolours and oils, culminating in a full-size copy of Snow Storm – Steam Boat off a Harbour’s Edge, which is just so wonderful. Even having studied it so closely, it’s still one of my favourite paintings. Look at it, and you can tell that he was on that deck, you can see he went to the top of that mast. You know he was there.