I first met Terry during book signing tours to Australia in 1996 and 1997. When there's a line of hundreds of people, you don't want to take up too much of the author's time, but he was polite and pleasant and signed my books - I think at the time, it was a limit of 3, so I ended up getting several books signed simply by going to multiple signings throughout the day. Many of my books have a dedication line ("Best wishes", "even more wishes", "a quantum of wishes", etc) but many more just have a signature. Terry also did an evening talk during those two tours so I got to hear him speak then as well. At one of the signings I meekly handed him my fan letter (I was an impressionable young spotty teenager, not named Kevin, at the time), and got a very nice posted response some weeks later.
The next time I met Terry was in 2007 at Nullus Anxietas, the first Australian Discworld Convention. I was fortunate enough to win a position in one of his klatches, and although I didn't get a chance to ask him my question directly ('What music do you listen to while you write?') due to other people being faster, I just asked Daniel Knight to ask it for me as he was interviewing Terry for another session at the Con later that day.
We had a Skype call with Terry at Nullus Anxietas 2 (2009), and then I saw him around at the Discworld Convention in 2010, although I don't think I directly spoke with him (although I did ask questions during the panels for most of the other guests - Stephen Briggs, Ian Stewart, Jack Cohen, Bernard Pearson, etc). Of course Terry did manage to come to Australia again for Nullus Anxietas 3 (2011) and in amongst all the other things he did at the Convention, I was lucky enough to be seated opposite him at the table for the Gala Dinner. Due to the noise, we didn't really get the chance to talk much, although he did ask about my costume (Cephut, God of Cutlery) at one point.
And of course, for Unseen University Convivium a few weeks ago, we managed to get a Skype call with Terry & Rob teed up, which was within my purview to organise! Although I didn't speak to him much on the call beyond getting it set up (I had just run a 2-hour quiz, after all), it was fantastic and I think all the fans there who wanted to ask Terry a question managed to do so.
Unseen University Convivium 2012
University of Adelaide, South Australia, 6-8 July 2012