It's not them who ultimately direct the voice actors, but the voice director at Skylark. All the shoddy KH voice work all boils down to them as well, since the language barrier with the Japanese correspondent on Skype has a negative effect on their output, especially with post-FFVII Cloud. Thankfully that person seems to have taken a backseat with KH3 and, it seems, FFVIIR.
*doof* Meant to include them, but I meant in terms of writing/how scenes are directed. For example, looking at Cloud's lines, appearance, and mannerisms in both KH1 and 2 give off the "cool, stoic loner" vibe. So in this scenario, people who don't know any better could look at that and think "he talks like that then." And poof! Bored emo Cloud is born.
Looking at another character. Demyx's dialogue and mannerisms are much more animated and lively. And with that came a more animated and lively voice direction.
EDIT: There's also how Skylark might be trying to copy the Japanese voices to varying degrees of success...
But also (it's been forever but I'm positive this was said) in an interview for Advent Children, it was stated Cloud acted the way he did because he was essentially hijacked by fan interpretations.
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I wanted to edit this in last night, but my power went out. ;~;
So in that sense, while the voice direction at Skylark has been ass many times, there may be other factors involved where they're not given enough to really work with. (Not trying to remove any blame from Skylark since they still chose what they did. I'm just trying to look at the whole scenario.)
I get people's disappointment but let's not pretend this is the first time that voice actors for Final Fantasy VII have been replaced. Sephiroth has had two voice actors and Aerith has had three. Most of them haven't done voice work for these characters outside of Final Fantasy VII Advent Children, which was almost 15 years ago, and the parts they have had have been really small or a part of a spin-off game.
Admittedly, I sometimes have a hard time counting Lance Bass since Sephiroth spoke so little in his appearance in KH1 he came off more as a placeholder.
While I do actually agree here, I believe most complaints about the voice changes are toward Cloud and Seph. Characters who show up all the time in everything, so people are much more used to and expect hearing Steve Burton and George Newburn as Cloud and Sephiroth than say, Liam O'Brien as Red XIII (whose voice is up in the air at the moment.) I also haven't heard anyone complain about Aerith. Probably because we're already used to her voice actress changing. lol
Briana White is such a precious bean. ♥