John Scalzi has blogged recently about science fiction and race here and here and in the comments he has received a certain amount of stick for not explicitly showing that he is writing about people who are non-white. I find it a bit odd that some people seem to think racial quotas are good in SF but I'm going to leave that because, like Mr S I'm an English-speaking, white, middle-class, university educated male, and have therefore won first prize in the lottery of life. Hence for some people I am incapable of imagining what it is like to be discrimminated against. [ I'll note that as a visitor and former resident in East Asia I have certainly experienced discrimmination but a) I accept it wasn't that bad and b) so what? ]
Anyway I'm not going to touch that debate any more because I don't see that it matters and instead I'm going to hit on a slightly ralated note. The question asked by John and his comenters is when you don't encounted a descriptive clue to tell you about a fictional character what do you perceive that character to be? One thing I notice is that I tend to give people English accents and get shocked when other versions are possible.
Example - Lois M. Bujold recently read aloud a bit of TSK: Legacy on a radio interview here. Even though a lot of the descriptions are distinctly American (the location is a sort of alternate Ohio) with my mind I still heard English accents. In fact I pictured Fawn speaking with an East Anglian kind of accent and Dag perhaps a slightly posher version of the same. And hence when Lois (who is from Ohio originally) read from an extract I wanted to say HEY THAT'S NOT HOW HE/SHE SPEAKS.
The same by the way goes for TinTin. The first few TinTin books I read (forget which ones) were in English and I naturally assumed that TinTin was an English journalist, Captain Haddock an English sailor etc. It was a major shock when in one of them (and I forget which one) this idea became completely untenable because TinTin drove to Switzerland (or something like that, I forget the precise details). Since Enlgand is an island you can't drive to Switzerland and that forced me to go back and figure out that he's actually Belgian. To be honest I think I got that wrong at first anyway and assumed he was French, but then I doubt I was 10 at the time.
There are plenty of SF books where I really have a hard time hearing the characters speak with an American accent - Honor Harrington for example or the humans in "The Mote in God's Eye" - and that is doubly true for fantasy. That may not be so surprising given that many fantasies are based on some sort of idealized medieval Europe, but it still causes cognitive dissonance when I am forced to hear them speak American...