The magic of Jim Kay’s art is in its mischievous detail.

The illustrator sneaks tricks and jokes into his drawings so surreptitiously, you can’t possibly see them all on a first viewing. I’ve scoured this man’s work many times, flicking through the pages of his illustrated Harry Potter and the Philosopher’s Stone. I’ve squinted at his paintings of Diagon Alley, trying to spot every sign, every creature, every visual pun.

To be honest, I thought I’d seen it all, but here I am, staring at an enormous framed print of Flourish and Blotts on the wall at the opening of Jim’s exhibition at Seven Stories in Newcastle. There are owls in the architecture, owls perched in windows and owls on turrets – you almost can’t tell where nature ends and stone begins.

For the first time I notice that wherever there’s an owl, there’s a stream of white bird poo beneath it. Yep, Jim’s drawn owl poo on the roof of Flourish and Blotts because, I imagine, he found it hilarious. Which, given that he’s also painted a few naughty body parts into jars in the background of Snape’s supply closet, is a recurring theme.

The main room of Jim’s exhibition is white with little winged keys and umbrellas painted onto the wall. Framed graphite sketches on thick cream cardboard, glass cases with early Hogwarts Express models and vibrant final paintings hang on the walls. A graphite-drawn Dumbledore peers over his half-moon glasses from one corner, next to a stern-looking McGonagall, in clothes inspired by the life cycle of a frog. There’s a Harry sketch, a Hagrid painting, a Hogwarts blueprint and a delicate mugshot of Ron Weasley from three angles.

Seven Stories Jim Kay Collage

Once we’ve wandered around that room, smiling and whispering to one another as though full volume speech would be disrespectful somehow, we go upstairs to hear Jim speak. The room is laced with fairy lights and decorated wall to wall with oversized cardboard cutouts of Diagon Alley shops. It’s nothing short of magical.

‘Diagon Alley is autobiographical for me,’ Jim says, addressing a small, enchanted crowd. ‘When I was a kid, my school friend had this sense of humour. I’d draw something and then he’d go in and add something funny or silly or rude. He’s no longer with me so I have to do that myself now.’

He spots a woman with her two young daughters in the front row and beams. Ten minutes ago, he’d met the little girls at the colouring-in table and appraised their work. The littlest, who must have been just four years old and dressed in a bright floral raincoat, drew Hagrid’s hut and then sat spellbound as Jim drew her a bat on the same piece of paper.

It speaks to a rather charming sweetness of character that Jim said a quick hello to the adults in the room before sitting down and hosting an impromptu sketching lesson for the kids in the room. It also might be one of the secrets to his work.

‘You give kids a pencil and paper and they just draw. It’s instinct, they don’t think about it or worry about it, they just draw. It’s sad that we lose that. I try not to lose that,’ he says.

Think about that owl poo on the roof of Flourish and Blotts. You haven’t lost it, Jim.

The Illustrating Harry Potter exhibition is open from 14 November 2015 until 17 April 2016 in the Gillian Dickinson Gallery on Level 6 of Seven Stories, The National Centre for Children's Books on Lime Street, Newcastle Upon Tyne. For more information visit the Seven Stories website.