When I joined them last Monday in Belgium, they had just shuttled down from Frankfurt, and were looking tired, though not as tired as they are going to be after Holland, Hamburg, Munich, Zurich, Dublin and several shows in the UK.
Both live in Massachusetts, Pailthrop a good-looking amazon and Williams, a tiny, frail and unassuming 30-year-old woman in flat boots and glasses. She is lethally bright and optimistically witty but, after a long drive and three local interviews - "I should just about be able to do this show before I collapse," she says - you wonder how she will muster the energy for the job at Brussels' Ancienne Belgique.
It is not a problem. On stage alone in the darkened club, she has unleashed her blonde hair, ditched the specs - but Miss Williams! you're beautiful - and looks rather like Elizabeth Montgomery from Bewitched. She says her guitar playing is "elementary", but it does not seem that way; her three-octave voice is sweet, full and best compared to Joni Mitchell's.
As for the songs - well, they can make you catch your breath. They deal with everything from friends to love to the evil influence of the Wal- Mart chain on the stateside community. As she begins a number, you can hear her whisper to herself: "Here I go". But that is not vulnerability; she is a droll raconteur and, by the end of the evening, the audience is roaring for more of her music, self-deprecating quips and Gilda Radner- style gurning.
That was Monday. On Tuesday we were in Pailthrop's silver hatchback heading toward Amsterdam, which should be a simple 200km skip.
Williams and I are on map-reading duty, but deep in conversation about Gary Oldman, who appears on billboards everywhere advertising clothes, when we miss a vital turn.
"Oh maan, girls!" roars Pailthrop, incensed. Back on track, we aim for Antwerp, which we should skirt on the ring road. We are following the last hotel's instructions, using a map bought from them; so why has the motorway they suggested disappeared? The map turns out to be 20 years old, and before we know it we are lost amid bakeries, bicycles and viciously hooting cars.
"Let's relax," says Williams, the soul of calm. "This gives us a chance to see if we'd ever like to come back to Antwerp." Someone bellows as we crunch across a tramline. "And I think the answer's no."
We emerge bound inexorably for Rotterdam, a serious detour when the sound check is at 4pm. Williams, however, keeps us entertained with highlights from her role as a singing potato in a student film, and we cruise down the E106 all joining in the chorus: "Hey! Mr French Fry, waddya know."
Unbelievably, we make Amsterdam's Paradiso Club with time to spare, and this is where Williams and I sit down to talk at length. It is also where it becomes clear that her equanimity has been a hard-won thing. Born in the New York suburb of Chappaqua, Williams was the youngest of three competitive sisters in a bookish household.
Left with the things the other two were not that great at, she took a religion and theatre course at college and, almost predictably, developed clinical depression.
It had to be her sister who noticed. "I asked her: `Um, does everyone think a lot about killing themselves?' And she said: `Oh God, we've got a basket case'."
Williams found herself in therapy; and it worked.
"Depression gives you the idea that your own agenda is not important, and it's amazing how badly you'll take care of yourself," she says.
"Not only do you feel inferior, you feel so unentitled to a normal life, you don't allow yourself the benefit of your usual coping mechanisms."
A grin. "So if I burn my breakfast now, I realise I have to do a lot of self-maintenance around not turning it into a grand trauma, part of the string of failures that has been my whole life."
These days, she is happy to send up her weaker moments and not worry if some people think she is nuts, because it might help others realise that they are not.
Resurfacing, Williams turned from theatre to her childhood friend, the guitar.
She sang in Boston's coffee shops, toured relentlessly and in 1995 brought out a CD, The Honesty Room, full of stories about punk angels, Mark Rothko paintings and one particularly tender track, "You're Aging Well", about a girl who repaints knuckle-rapping street signs on the road to old age with the message: "It always starts here...."
Mortal City, which appeared the following year, is deeper and more passionate. It features the standout track "As Cool As I Am", a crowd-pleaser which, when played live, has men applauding on their feet.
This is odd, because the track is about a girl whose boyfriend cannot stop mentioning the beauty of other women, including her friends - so she leaves him because he is making her hate them.
"That happened to me, but I wrote it for a friend," she says.
"Her man would say, hey, I'm just the kinda guy who likes breasts like this, or hips like this, and it's too bad you have that body, honey. At one point, he said: `well, you're not conventionally attractive', and that was bizarre, because she really was sickeningly beautiful.
"Then I met someone, and the same symptoms began. I told him a woman, a performer, was quite threatening for me and he said: `Yeah, and she is so sexy'. He said: `I wish you could find out the thing that she does, because she's really so alluring on stage'."
She laughs. "Oh Gaahd. It's like, why don't you find a way of being human? Then he said: `You know, I think it would be great if you were a little more aware of your clothes'.
"At first I thought, maybe this is the kind of tough love that's gonna help me, but my stomach was in knots. "Eventually I said: `I think we're gonna break up'. He told me: `I could not agree more, you're much too sensitive'."
She stops chuckling, sobers up. "But this guy, I broke his heart. See, how do you make a good-looking woman stay? You tell her she's ugly, so she'll think she can't get a better prospect. And it works the other way. Women do it, too."
There is a new CD, just out, called The End of Summer - a fuller-sounding outing, with a band, a rockier feel, and Williams's developing voice heading for Emmylou Harris territory. The honesty is still around; there is even a deft song about therapy - "Oh, how I loved everybody else when I finally got to talk so much about myself."
At this point, the door is thrown open, and Pailthrop marches in to say we have to move the van, and there is no hot water in the shower, and the mobile will not recharge. Williams puts on her glasses and says: "Fine, here's what we do...."
`The End of the Summer' is out now on Razor & Tie Records. Dar Williams appears at Bristol Fiddler's (0117-929 9008) on 30 Sept; Dublin Whelan's on 2 Oct; London Queen Elizabeth Hall (0171-960 4201/4242) on 3 Oct; Edinburgh La Belle Angele (0141-287 5511) on 5 Oct; Chester, Telford Warehouse (01244- 390090) on 6 Oct