You see, when I first watched him on TV I thought he was more famous than he was - kind of the opposite of that thing when you see someone from the telly in the street and start to say "hello" before you realise your mistake.
The suspicion crept up on me that we'd met.
There was this weird, fuzzy memory of an afternoon spent in Dublin in January 1995, where I was doing work experience. He was sitting, talking very fast in someone's flat - his own? - and was in the company of two other people, one of whom was on my newspaper course and is now a very well-known journalist. In fact, he's editor of The Times. I told you it was a weird memory.
So when there was a sudden downpour on Sunday at the Cambridge folk festival and I retreated into a tent only to find that the he was in there too, the responsible course of action was to ask, even if it laid me open to the possibility of discovering that I was losing it...
"Yes, that was me," he said, looking quite surprised, before adding that it was nice to see me again, thus demonstrating that he has good manners, since I know perfectly well that if I hadn't seen him on television in the intervening 17 years I probably wouldn't remember him either...
However, it was such a relief to find that my mind hadn't been playing tricks on me that I had to go and sit down for a little while before returning to suggest that since it was clearly going to rain for a long time would it be OK if I interviewed him? And since he was well-mannered (and trapped) he agreed.
It turned out he was there because he made a documentary, along the lines of reclaiming a forgotten love, about playing with Richard Thompson for Sky Arts last year. The marginalised object of desire was folk music. This year the channel, which media sponsors the Cambridge folk festival, had invited him back to be interviewed. He was also doing a guitar workshop with Martin Simpson and babysitting Simpson's daughter, who seemed very sweet.
Mangan swore faithfully that he wouldn't say anything interesting to Sky, leaving the field clear for me.
"I don't know why I like folk music," he said, "because my wife doesn't like it and my friends don't like it and it makes me an outcast in my own circle. But I'm from County Mayo in Ireland and parties at our house involved carpets and rugs being pushed back.
"The process of discovering folk was a lucky one that started in the late 80s and early 90s with John Martyn and Nick Drake, then had me stumbling from one thing to another. I went to America for a year when I was 18 and someone gave me a list of things to listen to. I started with Richard Thompson.
"Then in my early 30s I got into the hard stuff: Nic Jones and Dick Gaughan. But there's this feeling that because I don't have that many mates who are into it I'm like Uri Gagarin, heading out into the void. I'm here to make some friends who like folk music."
He took a bit of a conversational detour enthusiastically to praise Fionn Regan's song Dogwood Blossom, from the soundtrack of This is England.
"For some reason it speaks to me: maybe because I have a strong melancholy streak. I like depressing music but I'm an optimist who also likes storytelling. My wife is the opposite - perhaps we complement each other in this respect. She's a pessimist who likes dance music.
"So for a long time in our house folk was a four letter word that I dare not utter. It was desperately uncool - maybe it still is, but I don't care. Eventually you have to face up to who you are and I'm a folky.
"But I'm a folky with two young children and I'm in a play at the moment that's on nearly every evening," - Birthday at the Royal Court, in which he plays a pregnant man - "so I don't get to see films, plays or gigs apart from the ones I'm actually in. When I was asked, I jumped at the chance to come here again. I love nothing better than to bring myself to the point of vomiting with nerves in the company of Richard Thompson." Not that he'd have to do that this time around...
But standing on stage is what he does for a living. Why would singing with Richard Thompson make him nervous?
"If we were doing Othello it would be fun. In fact, I think Richard should give us his Othello at some point. But there's something very exposing about singing. The music is dictating to you - the song is its own lyric. Whereas with acting you're in control.
"Singing is the most popular art form in the world but it's totally about emotion, so it's very revealing. You're expressing something about what it is to be a human being in a very pure way. Which makes it all the weirder that I'd choose to stand on a stage and sing one of the most famous songs ever written - Who Knows Where the Time Goes - while the audience nudges each other and remarks that I look the donkey out of Shrek.
"But it's good to do things every once in a while that make you want to shit yourself with fear."
Why? Most people go out of their way to avoid that.
"It makes you feel alive - it was an amazing thing. I stood there at the side of the stage and heard Richard Thompson do one of the best sets he'd ever done and then say at the end of it 'And here's Stephen Mangan'."
Despite being from County Mayo, Mangan has one of those Irish accents that doesn't really sound like an Irish accent. Dick Gaughan, on the other hand, has some fairly pungent left-wing views. I wondered whether he was comfortable with that?
"Yes, I'm quite a lefty. it's hard not to be when you're an actor because you're dealing with people. They're your bread and butter, so you have to try and understand them."
I'm pretty sure Adolph Hitler thought he understood people.
"Yes, but I don't think Adolph Hitler thought of people as people. As an actor you are mainly about people and community, and what makes people tick. I always viewed left-wing politics as having a compassion to it: I deal with the fact that kings and dustmen are all on a level, they're all human beings. For my purposes I can't look at them from a status point of view. And once you're looking at everyone in the same way you're coming from an egalitarian point of view."
A bit of probing finds that he doesn't like Mumford & Sons and when I asked about Bellowhead - whom I love - he changed the subject. "Show of Hands. They're good. I just had a tweet from the Shrewsbury festival asking whether I'd like to go along this year because Richard Thompson is going to be there, so maybe I'll get to meet them.
"After the last time, Richard invited me to sing with him again in a general kind of a way. So for a while I toyed with the idea of turning up every time he played and saying brightly 'I'm here to do my song'. Now I'm thinking of faking an emergency appendectomy so I can go to Shrewsbury. We're going to be on a family holiday in Wiltshire at the time, which isn't too far away."
So if you're at Shrewsbury and you see Stephen Mangan, be nice to him. He's there to make some folky friends.
* You can buy tickets to see him in Birthday at the Royal Court here.
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