Having been to the recording of tonight's show I can say that Rock and Mangan provide the laughs, and Humperdink is smooth in conversation and performs his Eurovision song in his trademark powerful voice. Stewart is the biggest name on the sofa, but she is also the most disappointing for me.
I've always found Stewart awkward to watch in interviews. She seems uncomfortable, and seems to have nothing to say, which makes me uncomfortable, and then I find it a bit boring because of all the stuttering and pauses. It can't be pleasant to have to be the centre of attention all the time, but it is her job, and sometimes I wish she'd just get over it.
It turns out seeing her live at the recording of a television show was no less uncomfortable than watching her on television, but Graham Norton, with his consummate skill, makes the experience a little better. She has some decent stories, and on tonight's episode will talk about filming love scenes for Breaking Dawn, the last in the Twilight trilogy.
The story is about how the censors are pretty strict, and cut out things like "sounds" and thrusting - from what I could work out it was probably her co-star and real life boyfriend Robert Pattinson doing the thrusting.
She says: “It is a bit full on but it’s edited. I see scenes and I think why is music playing over that, I was definitely making sounds and it could be so sexy and it’s not.
"The ratings’ board breaks it down so technically and apparently, thrusting is a huge problem for the children."
It's a fun story, but it would have been better if it had just flowed out of Stewart's mouth the way lines do in the films she's in, rather than being spread out between awkward pauses, lots of uses of the word "um" and constantly turning her face away.
Luckily, the other guests on the sofa save the night. Rock has some great tales to tell involving his daughters, and the fact that he's sort of friends with President Obama. And the discussion becomes serious when Norton asks Rock for his views on Obama's decision to come out and say he supports same-sex marriage.
Rock also speaks about his latest film 2 Days in New York, which looks really, really good. He takes on the male romantic lead, instead of the funny sidekick or the comedy zebra (Madagascar), and from the clip that was shown, he pulls it off.
Mangan has been doing the rounds recently to promote the second series of Episodes. I saw the first few episodes of season one, and really enjoyed it, but never got round to finishing the season. I'll have to catch up and watch this second season as well, since I really like Mangan both in character and in real life.
But, there is bigger news than Episodes. It turns out Mangan is playing only one of the most famous characters in Britain, who everyone grew up with. I don't want to spoil the suprise as to who it is, but here's what Mangan says about him on The Graham Norton Show: “He is the least sexy character ever written. I have played him and Adrian Mole and my wife says, ‘who have I married?’"
And so to the final guest, Humperdink, who comes on all tanned and with that fabulous bouffant hair (what shampoo does he use?). Norton will be hosting Eurovision again this year, and the two have a good chat about the competition, with Humperdink saying he's honoured to have been asked to represent Britain. The song, Love Will Set You Free, is a mournful ballad (it's a Viennese Waltz apparently) and in the pantheon of Eurovision songs, it's actually pretty impressive.
Which is a good phrase to use to sum up tonight's show - pretty impressive. Rock, Mangan and Humperdink are all entertaining, and Norton manages to bring out the best in Stewart and create a seamless conversation despite her awkward manner. Douze points.
The Graham Norton Show is on BBC One tonight at 10.35pm.