Sometimes you just like to spring the occasional impossible question on them.
This one comes near the end of an engaging chat with Elvis Costello. In the best part of an hour on the phone from somewhere in America, the veteran English songwriter has led a discussion on his elegant album with Burt Bacharach, Painted from Memory; his ever diversifying career; the trick of ageing gracefully and still playing the songs penned as an angry young man 20 years ago.
The last is something we'll be able to see up close.
Having played the first Sweetwaters in 1980, Costello returns to the resurrected festival in a month.
But back to that question: what would the leer-and-spittle Costello of the New Wave years think of the Bacharach-collaborating, classical-dabbling Costello of 1998?
"I have no idea," he fires back. "What is the point of that thought? I think I was a very nauseating puritan a lot of the time, who also was having the time of his life being as rude and drunk and lecherous as possible — and isn't that right at 23?
"You are all of those conflicting things and probably very, very intolerant. But I had a deep love of Burt Bacharach's music then, otherwise I wouldn't have been playing it. It meant the world to me.
"Dusty Springfield is, if not my favourite singer, among the five or six singers who matter the most to me. Her relationship with Burt's music alone would have made me love it.
"I couldn't have conceived of doing it then because I was too busy with the things in the moment. It was just the way you are at the age. Neither better or worse."
In the years since emerging bespectacled and bitter with his 1977 debut My Aim is True, Costello has created a remarkable, often remarkably confusing musical career.
He has never had runaway commercial success. But to be a Costello fan was to expect a new challenge every album — and know there would be at least a couple of masterful songs even if the rest was unsatisfying.
Having hit his increasingly eclectic stride in the 80s, he could rarely be accused of repeating himself — though he did reunite with his original backing band, the Attractions, in 1994 for the album Brutal Youth and a tour.
But fronting a rock combo doesn't sound like something the 43-year-old ever wants to do again. In fact, he's quite happy to be off the album-tour-album treadmill having parted company with Warner Brothers after 10 years on the label.
Now, he says, there are two or more careers going on.
There's the one connected to Painted From Memory — he's just finished a short string of live dates in the states with the 70-year-old Bacharach and backing orchestra.
"I came out of a concert in Chicago about six weeks ago and we were just mobbed," he laughs. "Burt and I, we were mobbed by people in their late teens and early twenties and it was really strange."
There is touring with Attractions keyboardist Steve Nieve. The piano-and-guitar approach is what he's bringing to Sweetwaters. The duo makes it easier to perform the back pages of his songbook without trying to replicate the arrangements which tie the tunes to the past. And they can throw in a few Costello-Bacharach numbers, too.
"We've found that is the most direct and flexible way of playing a huge repertoire, because there doesn't seem to be the difficulty in turning corners from a 1977 song to a 1997 song that there would be if there was a band on stage."
Costello's partnership with Bacharach is but the latest in a list of get-togethers over the years which have included Paul McCartney (a few songs), the Brodsky Quartet (the baroque-pop Juliet Letters album, sort of Costello's cello years) as well as jazz guitarist Bill Frissell, Brian Eno, gospel group the Fairfield Four, the Mingus Big Band ....
The latest collaboration began with the song "God Give Me Strength," which Bacharach and Costello wrote for the soundtrack of Grace of My Heart, a half-realised film loosely based on the life of Carole King and the changing American pop scene of the 60s.
That turned to plans for a full album with Costello's singing framed by Bacharach's subtly symphonic arrangements.
With Costello's long-held affection for Bacharach's exquisite 60s ballads, might not this been a studied exercise in emulating a pop classicism of the past?
"No, because I don't think it is vintage. There is a timeless quality to the sound of a pop orchestra which we weren't afraid to use ...
"It really seems to me this record happened very much in the moment. The fact that it doesn't sound like anything else allows you to think it comes from the past. But it couldn't have been written then — because I wouldn't have been on it."
It seems the Bacharach catalyst has helped Costello to pen one of the most affecting albums of his career, due to the melancholy mood throughout and his heartfelt singing, ranging from intimate torch tones to lung-busting crescendos.
"It was largely about lost love, and we'd stay within that world because there is plenty to be said on that topic and I think the album proves it."
Painted from Memory has already achieved a profile higher than most of his recent efforts, though he says it should also prove to be an album with a long shelf-life.
"It obviously doesn't compete in terms of, `That has got to be on the radio next week otherwise it's a disaster.'
"This is about people discovering it and finding a place for it in their lives. It's grown-up music ..."
Costello has some other grown-up music on the go. Before coming here he heads to Oslo to perform with the Swedish Radio Symphony. Just an old tune he had kicking around originally written for antiquated classical instruments. As you do.
"It's a song I wrote a couple of years ago which was written for viols, the Elizabethan instrument, and now I have written a full orchestral version of it."
But it's not all highbrow songcraft. Another reason he's in the United States is to make a guest appearance in the next Austin Powers movie. Bacharach was in the original.
And later next year there may be more concerts with his venerable co-writer and maybe another album.
But first he's got to return to Sweetwaters, 19 years on. No, he doesn't remember much of headlining the first one.
"I remember the journey more than anything else. I remember taking the band on a round-the-world trip. We more or less spent the fee just going on that trip. We flew to Singapore on the Concorde, then we flew to New Zealand. I think we probably enjoyed the trip more than the concert. Does anyone remember it? Have you found anybody who will admit to having been there?
"I don't know how good the show was, it was probably pretty ropey. I do remember sitting on the beach in New Zealand in my suit in the sweltering heat and people coming up to me like I was really weird. I would never loosen my tie in those days. That is the kind of guy I was."