In this interview with JK Rowling, she describes how she imagines the voices of some of her characters (Hagrid, McGonagall, Dumbledore, and Harry):
Lydon: Now I want to hear J.K. Rowling herself do ...
JKR: Do Dumble-, er - not Dumbledore - Hagrid!
Lydon: Either one ...
JKR: Well, Hagrid's sort of West-country - yokel - which is where I grew up - the part of Britain where I grew up, I didn't grow up in Scotland, I grew up on the border with Wales. So Hagrid's kind of ... 'Yes Harry Potter, sir' - like that - very slurred words - it's the accent English people always put on to sound stupid ... [laughs] Hagrid isn't stupid, but he's got that kind of very country - you know, way of speaking ...
Lydon: And how about Minerva McGonagall?
JKR: Very clipped, and very, very - quite upper-class and very brisk - like a governess [Lydon laughs] I - I - I can't do it, but I kind of see Dumbledore more as a John Gielgud type, you know, quite elderly and - and quite stately.
Lydon: And Harry himself?
JKR: I suppose he sounds like me. I always do my voice for Harry, when I'm reading to my daughter.
The audio of this section of the interview, including a minute or two of JKR reading aloud from her own book, is available here (Internet Archive; sadly the original link has gone dead).