They've almost all been graciously appreciative. Even if it's just a brief, standardized thank-you note, the people on the other end of my fanmail have seemed truly pleased that I've taken the time to write.
Today, a webcomic that I follow had a line that made me laugh, so I sent off an email to the artist stating that I found that comic particularly amusing. She's had a running bit lately about the many people who say that if they had any money, they'd buy her books, so I make a joking reference to that. Five minutes later, I got a reply. She suggested I buy her books. Words like "thanks," "appreciate" and "grateful" didn't appear in her message at all.
Now, I'm reasonably emotionally differentiated, so I don't find this crushing, nor will it greatly diminish my enjoyment of her work. But it did make me think about how public perception and image can be quickly changed by carelessness. I said "I like your work" and she said "Then why don't you give me money?" Perhaps it's too sore a subject to joke about (although she's been using it for material), or maybe she's just having a really bad day. I don't know. It was hardly an ideal celebrity/fan interaction, though.
As someone who wants to build a fan base, I don't ever want to make this mistake. I hope to never cause a future fan feel unappreciated, belittled or guilt-tripped. It's my job to make my work so appealing that the fans can't resist buying it, not to push them into it. And certainly when someone praises my work, I want to clearly express that I'm very, very grateful!
When I am famous, every day will be Fan Appreciation Day!