Honestly, I'd just appreciate it. In my head I know how my characters look to me. But part of reading is the fact that every single reader gets to imagine it in their own way. Sure, the same events will unfold, and the characters will have the same details to everyone, but if any author ever says someone's interpretation is wrong, then they shouldn't be writing. Because that only means they're doing a poor job at getting the description across.
Descriptions are meant to be loose enough for an entire audience to describe any given character in a story as a different person than what the next reader imagines. Otherwise, your looking for a different medium to get the story, like a comic or motion picture. And at that point, it isn't a description.
I mean, missing important details, like a tattoo with a lot of meaning, or teeth-mark scars along the arms, could make the character come off as "wrong." But sometimes details can be forgotten, or someone just might not want/know how to draw a specific detail. Even then, the readers interpretation comes straight from the writing. And I can't imagine any author of any work being upset with fanart.
(Also, confidence is a tough thing. Never let it hold you back if you wanna do fanart, because any writer with good writing wasn't confident, and felt afraid to show their novel in the first place. So the author can sympathize with the artist when it comes to confidence in one's own work. Anyone who thinks they're the best and then shares their work is either going to feel really upset soon, or they're foolish and won't grow, because they'll bash on feedback and criticism while demanding compliments, and that will... also lead to disappointment, because no one will like their works or their personality.)
You wanna draw someone's characters, just do it and make their day! ^.^ Maybe their response will make yours, too!