I can't say I ever pictured Twoflower as the 'typical' Chinese/Japanese tourist...I think it's because I read the copy with the awesome Josh Kirby illustrations in which Twoflower was a little fellow in bright clothing and literally had four eyes and I think that image stuck in my head while I was reading.
Plus, I never really 'see' characters when reading, anyway, except for the very, very loose details (I think I think in verbs -- tend to 'see' movement and a little of the setting instead
) so I guess I shouldn't keep harping on about characters looking like I imagine them -- as long as the actors do a good job and the films recreate the magic of Discworld, then I'll be happy.