"I'll just be a couple of seconds, I've got a patient in here," says Sir Paul McCartney, poking his head round his office door.
He disappears again, leaving you to nervously scan the books in the waiting room - three dictionaries, some Taschen art folios, a few dozen Beatles reference works and a surprising number of Jamie Oliver cook books.
Minutes later Sir Paul reappears, ushering out his "patient", who turns out to be one of the many assistants at his plush, five-storey office in Soho.
"Take the pills three times a day," he quips. "You might feel a little drowsy at first."
As he beckons you in, the star begins fussing around the room, making small talk ("You're from Belfast? I thought I detected that lovely little accent") and straightening up his picture frames.
"Slight obsession," he says. "What do they call it? Obsessive compulsive disorder. Disorder! I think it's the opposite. I think it's order."
All this kerfuffle is designed to put you at ease.
The most famous musician on the planet is fully aware of the effect he has on lesser mortals and, 56 years after Love Me Do, he's become adept at soothing people's nerves before they collapse like a bouncy castle in a power cut.
You can see it in practice on his recent episode of Carpool Karaoke, when the 76-year-old is mobbed by fans outside his childhood home in Liverpool.
Slowly but purposefully, he walks through the crowd - shaking hands, saying hello, acknowledging everyone; but constantly moving forward until he reaches his car and, with one final wave, slips inside.
It's a finely-tuned survival tactic, forged in the early days of The Beatles, "when it first started off and got crazy".
"Before that, we just wanted fame and we didn't have any," he recalls in his office.
"Once we got fame, that was nice, then it got a little bit crazy, so you had crowds that were pushing and grabbing at you.
"What happened occasionally was you had security guys who would growl, 'Get out of the way! Move out of the way!' and the crowd would get a bit aggressive.
"So we learned to keep moving forward and be very quiet and calm about it - out of necessity, to not make a crazy scene any crazier."
Incredibly, though, the star says he is able to pass unnoticed in public.
Last weekend, before playing a concert in New York's Grand Central Station, he walked around the concourse, filming on his iPhone "just to get a feel of it" without being bothered.
And, on the rare occasion he takes public transport, he says people are often too distracted by their phones to spot him.
"If I'm on a train, I see everybody else just looking at their screens," he says. "But I'm looking out: 'Oh look, there's the London Eye! There's the River Thames!'"
And if a fellow passenger looks up, they'd presumably go: "Oh look, there's Paul McCartney!"
"Yeah," he laughs. "And sometimes they do look up and they take a little picture and they sell it to the newspaper.
"And this is the price of fame."
All this activity is in support of a new album, Egypt Station, which contains some of Sir Paul's catchiest, most immediate songs in years - from the blissful domesticity of Happy With You to the frisky Come On To Me, with its lyric about a hook-up at a house party.
"You know, rock 'n' roll, its origins are pretty funny and fun," says Sir Paul. "It's not supposed to be a serious job.
"Why we loved it was Elvis Presley singing Baby, Let's Play House, and he got it from the blues singers, who were all quite frisky, too."
Come On To Me's flirtatious lyrics were triggered by a romping guitar riff Sir Paul had been playing around with.
"You always start with a little guitar phrase or something," he explains. "I had the little riff that starts the song and I started thinking about parties in the '60s where you would see a great-looking girl and she'd smile and you'd think, 'Oh, is that a come on or am I misreading this?'
"Now, that doesn't happen these days I hasten to add! More than my life is worth. But often with songs you remember a particular event and you go back to it and you paint the picture based on that memory, so that is one of those."
Other songs take a more serious tone - such as Who Cares, which addresses the brutality of internet bullying.
"I was actually thinking about Taylor Swift and her relationship to her young fans and how it's sort of a sisterly thing," says Sir Paul of the song's origins.
"And I was imagining talking to one of these young fans and saying, 'Have you ever been bullied? Do you get bullied?'"
"Then I say, 'Who cares about the idiots? Who cares about all this? Who cares about you? Well... I do.'"
But the album's angriest moment comes on Despite Repeated Warnings - a diatribe about climate change deniers, with a lyric that couldn't be more timely or relevant: "Those who shout the loudest/May not always be the smartest."
"People who deny climate change... I just think it's the most stupid thing ever," says the star.
"So I just wanted to make a song that would talk about that and basically say, 'Occasionally, we've got a mad captain sailing this boat we're all on and he is just going to take us to the iceberg [despite] being warned it's not a cool idea.'"
That mad captain, could it be anyone in particular?
"Well, I mean obviously it's Trump but there's plenty of them about. He's not the only one."
Bond on the run
At the time of writing, Egypt Station was set to knock Eminem off the top of the charts, giving Sir Paul a 23rd number one album (including 15 with The Beatles and two with Wings).
So how does he measure success now? Are there any more records to be beaten?
"Success is if I like what I've done," he says. "That's the real one. If I actually think, 'Well, that's pretty good.'
"If it gets in some sort of chart somewhere, that's a bonus - but that's not as easy as it used to be.
"I remember one of my kids, when they were little, said, 'Daddy I like this song. Can you put it in the charts, please?' and I said, 'Yeah, I'll try my best!'"
A foolproof plan for a hit single would be to write the next James Bond theme - a bombastic follow-up to 1973's Live And Let Die. But the star is having none of it.
"One is enough!" he exclaims, before making the extraordinary revelation that "I never thought Live and Let Die was a particularly good Bond theme".
When it's pointed out that his perpetually-shifting, pomp-rock classic regularly tops polls of the best Bond songs, Sir Paul relents.
"Well now, you see, this is what happened. People say, 'That's my favourite!' So I'm persuaded and now it is my favourite too.
"It was on the telly the other day, that film, and I thought, 'Oh yeah, it works great.' But I think once is enough."
He'd make an exception, though, if Daniel Craig invited the Foo Fighters to score his double-oh-swansong.
"They'd be very good actually," says Sir Paul. "I think they could handle it. And then they could get me in on the drums!"
In any case, he's too busy to record new music, Bond or otherwise, with a mammoth world tour that kicks off next week in Canada and runs until summer 2019 (with a gap for a much-rumoured Glastonbury headline slot).
And while his contemporaries Sir Elton John and Paul Simon are engaged in farewell tours, Sir Paul can't imagine bowing out of the business.
"Retire from what?" he told 6 Music earlier this year - while admitting he understands the impulse to step away from "the actual physical thing of touring".
"Billy Joel does a residency at Madison Square Garden which is a nice easy hop from his house. I think people are starting to do that a bit more," he says.
What did he make of Joel's recent admission that he'd given up releasing new music because "I couldn't be as good as I wanted to be and I got frustrated"?
"These things happen to you. You get disappointed in what you're doing [and say], 'I'm never doing this again.' I suppose he's sulking, really."
If that mood descends on him - and it does - "I just go and do something else, maybe go on holiday or something, and that can get you hungry again.
"But I still love what I do. I know that's what everyone says but it's true. The idea of having some time to just pick up a guitar and noodle around and find a song in there, it's still magic to me, the whole idea of that.
"And it still thrills me when I get to the end of it and I've got a song. So I'll keep doing it forever."
Sir Paul McCartney's 17th solo album, Egypt Station, is out now.