The Observer offices are tucked securely behind the earlobe of the Guardian building. It’s a comfortable collection of tea cups and computers, with books piled precariously next to the odd well-intentioned plant. It’s not unusual for famous people to come into the office, whether to be photographed or interviewed, but it is unusual for staff to bother looking up from their screens. I mean, I don’t want to boast, but I have taken the lift with models. Politicians stalk the corridors. There was a memorable afternoon when someone’s neighbour wanted to learn about sub-editing, and after many hours of her studiously practising InDesign behind us, we realised it was PJ Harvey. Celebrity guest editors have come and gone, sometimes shyly, sometimes with an entourage the size of Europe, but in my many years sitting in this chair I’ve never seen the air change the way it did when Stormzy visited.

We picked him up from reception, 100ft tall in his black Ugg slippers, and his first request was to see the canteen (“I love school dinners!”) where, as he waited for his fish, chips, beans and chicken slice to be plated up, the room filled with people, all suddenly excited for lunch at quarter to 12. Somebody gasped as they passed him on the stairs and someone else stifled a scream. He brought his fish and chips up to the editor’s desk, where it quickly became clear we were going to have to keep the door closed to prevent strangers fainting.

At first the onlookers were colleagues passing politely, doing a special walk that suggested they were on their way to the loo, but quickly all pretence was lost and people came dashing down from departments across the building. A small crowd gathered, and then a large one, spanning ages from 20 to 70, in their eyes an awe.

After a polite request to leave our editor to his work, choosing between covers, checking proofs, they stepped back from the room, to return a minute later steely-smiled. What is it about Stormzy in particular that these fans love? “He’s not just a person,” said one young woman, wielding her phone in case he walked past again, “He’s a movement.”

With that in mind it was vaguely intimidating to re-enter the room and sit beside him as he discussed cover lines. He had forgotten how to write with a pen, he realised quickly – he hasn’t used one since he got his phone. Chuckling, he practised on a proof. “Hello,” he wrote carefully, “My name is Michael,” before ticking off pages. “This looks sick,” he grinned approvingly. “This is HARD.”

Does this always happen I ask, as he strolls across the office later to meet the designer and choose font colours, and grown-up politics reporters squeal and swell in his wake. “A bit,” he grins, bashfully. “Is there a… quiet way out?” his PR asks and, once he’s signed off the magazine, we weave past the picture desk to a back door, where he can dash down to his car without smiling for any more selfies. When he leaves by the loading bay exit a fire alarm goes off, and it’s the most peaceful the office has been all day.