For over 40 years, he’s been a Doctor, but really Mac Rebennack – the 71-year-old musician who performs as Dr John – is rather more stately than any normal physician. His hat at a jaunty angle, his walking stick rattling with eclectic souvenirs, a cheroot at his lips, he seems more like a visiting dignitary from some incredibly exotic foreign clime.

Which isn’t really an incorrect impression. A man with deep roots in New Orleans music and also the city’s spiritual culture, Dr John’s worldview is as unique as his life experience – ex-junkie; ex-con; ex-guitarist – is alarmingly wide. At our meeting in his central London hotel, Mac is a man entirely on his own wavelength but who happily receives any enquiry on that frequency.

For all he is a good interview, music is how Dr John communicates, and it’s a language he’s been evolving since he started out over 50 years ago. “I was taught as a studio musician to be aware, and to play all music the best that you could play it, whether you’re familiar with it, or even if you don’t like it, play it to the best of your ability,” he says today. “And that’s a code I still live by.”

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When we were recording The Sun, Moon & Herbs album in London, you put a gris-gris cross on my head. What is it, and what does it do?
Bobby Whitlock, Derek And The Dominos
Well, it’s a blessing, and any blessing is good. But, for Bobby’s information, I thought it would be a good idea for Bobby: he was playing with Eric (Clapton)’s band when they was Derek And The Dominos. I thought Bobby was a good singer and a good musician, but he just needed to have something called… perseverance. Because I could see he was going two ways at once.

How did you end up in prison?
J Gerard, via email
I was arrested for narcotics. In those days, there wasn’t rehabs or the places you can go today. In some ways I think I was blessed to go to a federal joint rather than a state joint, because they treated you better, though I can’t say that for a fact, because I never went to a state joint. Listen, I’ll tell you this. Prisons are not made for the betterment of people – all they teach you is how to be a better criminal. There are more people in prisons in the USA than there ever was, and that’s mainly because of drugs. It’s not a smart idea, to put people in drug culture in prisons. There are alternatives.

How do you rate Hugh Laurie’s piano playing?
Phil Newman, Brighton
I thought he played good. My little friend here [assistant/girlfriend, Susie] turned me on to Hugh Laurie – her friend knew his friend and whatever, and all of a sudden, I went to do a record with him in LA. He sent me a recording with Bessie Smith singing a soul song, and I cut it [for Laurie’s debut album, 2011’s Let Them Talk], and it was cool, and he played piano on it and that was cool, too. When things happen like that, it’s spiritual.