Its not regional, that’s true everywhere. Having lived each in China, South Korea and Thailand, much like Gar, I can tell you there’s no meaningful difference when it comes to physical appearance. Most Asians do get upset when people are culturally insensitive (not just Asians, of course), but there’s a difference between saying “Are you Chinese?” and finding out they’re Korean, and saying “Hey man I don’t speak Japanese” to someone who’s speaking Chinese, let alone actual slurs. Also, when discussed in a polite way, most East Asians will grant that if you took away the fashion, hair styles and makeup, it’d be impossible to say with any certainty where someone comes from in the area.
Getting someone who has the right nationality for every role is almost untenable. I’m sure there’d still be some people who moan, but unless the character is culturally significant (think casting a Chinese person as a geisha in a pseudo-historical film, although memoirs of a geisha actually had casting woes that are worth a full separate conversation), it’s a bit beside the point. A situation like the Korean characters in Lost is a fair example, also. If they had cast actual Chinese people and forced them to speak Korean, that would’ve been a travesty. The scenes set in Korea were already bad enough! (they terribly misrepresented Korea… one example being their wedding scene, set at a Japanese temple). Actually, Koreans mostly ridiculed Daniel Dae-Kim at first for his horrible Korean in the beginning of the series, and for his physical appearance not really matching the Korean standard of beauty (most Koreans would say he’s downright ugly), but they kinda grew to love him as the series went on and his Korean got noticeably better. So even then! Korean-American, and there’s still issues