It would be easier to list the celebrities who haven't appeared on The Graham Norton Show than those who have.

The BBC One chat show is a crucial stop on the promotional tour for stars who are launching a new film, book or album - but you also get the impression it's the one they most enjoy.

Jessica Chastain made a point of saying it's her "favourite show" as she sat down on the famous red couch earlier this year. Matt Damon declared his appearance "the most fun I've ever had on a chat show". Norton's is the sofa of choice for Will Smith, Dwayne Johnson and Tom Hanks whenever they're in town.

But perhaps the key reason for the show's success is the combination of the celebrities who appear. It's hard to forget June Brown and Lady Gaga, or Miriam Margolyes with Will.I.Am. The chemistry (or lack thereof) is something that often takes Norton himself by surprise.

"Usually you don't know what's going to happen," he tells BBC News. "You can only cook it so much, you line up those ingredients, and you don't know whether you're going to poison the nation or delight them with the lovely confection!

"Those combinations on the couch, they're pre-determined or pre-mediated to an extent... but there's an alchemy on the night that you just can't predict."

Norton shot to fame as a stand-up comedian and actor, appearing in three episodes of Father Ted. He got his first taste of hosting a chat show in the late 1990s when he stood in for Jack Docherty on his late-night talk show on Channel 5. Norton's talent as an interviewer and chat show host was obvious, and Channel 4 promptly offered him his own show.

So Graham Norton, which launched in 1998, was later extended to five nights a week and renamed V Graham Norton, such was its popularity. While his later attempt to crack America failed, he remained resolutely popular in the UK.

Norton moved to the BBC in 2005, and The Graham Norton Show launched two years later. It was promoted from its initial home on BBC Two to the prime Friday night slot on BBC One after Jonathan Ross left the corporation.

"The Graham Norton Show is a master class in celebrity pairings, offbeat humour, big red chair hijinks, and just all-around fabulous times," wrote Devon Ivie in Vulture, but added that not all episodes translate well overseas.

"The celebrity pairings are always fun, although they can lean a bit toward UK-centric faces if you're not well-versed in that culture."

Image caption Norton praises Dame Helen Mirren (pictured with RuPaul and Jack Whitehall) for "making other guests on the couch better"

Norton's programme, in its current format, broke with convention when it first launched by having all the guests on the couch at the same time. It meant they could interact with each other, as well as him.

"Helen Mirren makes other people on the couch better," Norton says as an example of someone who creates a positive dynamic. "I don't know what it is. I think it's because she's interested. So they'll start telling a story and she'll turn and gaze, and they'll up their game, because 'Dame Helen is actually listening to me, I'd better up my story'."

Stars who appear on the show usually have briefing chats with researchers, which Norton is given in advance so he can steer the conversation in certain directions.

But things don't always go according to plan.

Mark Wahlberg famously appeared to be drunk on one of his appearances, which The Guardian referred to as a "car crash".

And sometimes the celebrities just don't gel. Jamie Lee Curtis and Jeff Goldblum traded thinly-veiled barbs at each other during a rather awkward encounter last year, while Matthew Perry looked gloriously uncomfortable sitting next to Margolyes as she recounted a sexually-charged story about Laurence Olivier in 2016.

But Norton recalls one particular highlight - or lowlight - when it comes to guests clashing.

Image copyright Getty Images Image caption Norton has won several awards - including Baftas and NTAs - for his chat show

"Alex Kingston was very good with Rob Lowe. She didn't like him, and I really like Alex Kingston because she did not hide that," he recalls. "And she called him out on a couple of things. So sometimes you get a little bit of tension there."

(Kingston accused Lowe of having a huge ego in the episode, aired in 2011, highlighting the arrogance involved in him calling up MTV to ask for the phone numbers of girls he liked from music videos.)

"And sometimes," Norton continues, "you get guests who don't have the 'wanting to be liked gene'. So they don't care if the other guests don't like them. My whole life is a big wanting-to-be-liked gene, so I find it fascinating when you meet someone and it's like 'wow, you're in the public eye but you don't care if people like you or not'."

Plus, he adds: "People have to be so careful. Poor old Jennifer Lawrence, every time she comes on, she just says something, and for some reason she gets in such trouble and has to apologise to the people of Hawaii. And it's like, for god's sake, she's a very young girl, and she's being held up to this ridiculous standard."

Given its influence, it's often joked that a celebrity's placing within The Graham Norton Show can be used as a barometer for their level of fame (see the tweet below).

The chat show landscape in the UK is considerably different to the US, where practically every major network has a nightly talk show. Jimmy Fallon, James Corden, Jimmy Kimmel, Stephen Colbert and Seth Myers all present programmes which each run at least four nights a week.

In the UK, the major evening talk show options are limited to Graham Norton and The Jonathan Ross Show on ITV, both of which go out just once a week, each generally attracting between two and three million viewers.

On the plus side, that means both attract the cream of the celebrity crop. But the role of the chat show host itself has occasionally been called into question.

Suppose you have a massive Hollywood movie star on your couch. Do you keep the chat polite and non-intrusive, or ask them the uncomfortable questions a hard news journalist like Jeremy Paxman might ask?

"I'm so not going down the hard journalistic Paxman route," Norton laughs. "The big tell-all interview will never be us. I'm not interested in doing those, that's somebody else's gig, that's a different show, a different agenda - and it's a very difficult booking!

"I want our guests to have a nice time, so once we've agreed to have them on, we want them to have a nice time."

Image copyright Getty Images Image caption Norton had to address an elephant in the room when Mel Gibson was a guest

There are occasions, of course, where an elephant in the room needs to be addressed.

"The more difficult ones for us moving forward are going to come out of #MeToo and all those sorts of things," Norton says. "When we had Mel Gibson on, I didn't want to talk him about the comments he made, the accusations, the police reports or any of that stuff... I really don't think that's our show.

"But I do feel we have a duty to our audience to acknowledge that we're not stupid. Like, we do know. But it was a big deal, it's ridiculous, we were talking about Daddy's Home 2 [the film Gibson was on to promote].

"We agreed to Mel ages and ages ago, and as it got closer, it was right at the height of all the accusations, not about him but about other people, and we just felt suddenly really uncomfortable. We can't just have this man on and be all happy-clappy and pretend a bad thing never happened.

"So I think we just asked him a question about, 'Given what you've been through, are you surprised you've been allowed back into the fold?' and that was all it took."

In addition to the actors and pop stars, Norton's guests occasionally come from the political arena. He's had Hilary Clinton on twice in the last two years, as well as Labour MP Harriet Harman.

Image copyright Getty Images Image caption Harriet Harman and Hilary Clinton are among the few politicians Norton has welcomed as guests

The booking is done by his team, and guests aren't necessarily signed off by Norton. "Harriet Harman was an interesting one, because she was a serving MP, and I don't think serving MPs should do the show," he says. "I think they should have something better to do.

"If they're going to be on a show, they should be on Newsnight or Question Time, where they're going to be grilled about their job and what they're doing, not telling us a story about being a hopeless waitress or that time they met the Queen - that's showbiz nonsense."

But while right-wing politicians may be noticeably absent, Norton points out: "When Boris [Johnson] was mayor [of London] I think we asked him to be on quite a lot, but he never wanted to do it." He largely leaves guest booking to the show's producers, adding: "I rarely veto people."

Norton recently indicated that he'd like to reduce the number of episodes per year to allow him more time to "write a book and walk the dog".

He remains busy with his other presenting commitments, including the Bafta Awards, the BBC's Eurovision coverage and his weekly show on Radio 2.

For now, however, The Graham Norton Show remains a staple of the TV schedules, and it's easy to see why. It's energetic yet also relaxed. Glamorous, yet accessible. It's appointment-to-view television which has, over the course of more than two decades, become a chat show blueprint.

This is the second half of our two-part interview with Graham Norton, which took place last year. The first part of our conversation with him covered issues such as the BBC's star salaries list and the discipline involved in writing novels.