The only time I saw him was at Worldcon 2005 in Glasgow. I first glimpsed him queuing patiently, way down the line outside one of the more popular panels, on sex in YA fiction. A con newbie, I remember being impressed at the democracy of it. He was much in my mind anyway at that time, having just publicly slapped down J. K. Rowling for expressing her dislike of fantasy - an act of which I thoroughly approved.
He took his place in the audience for the panel, which got on with the important task of discussing YA sex. For one panellist, YA simply wasn't explicit enough. It was too oblique, too "tasteful" in its descriptions. "Sometimes," she complained, "you're even not sure whether sex has taken place at all."
"My wife often says the same thing," came a voice from the audience.
That, of course, was Terry Pratchett.