Come'on
Writers doesn't determine how the actor should coordinate with the suit actor. It is the director's job.
For Wataru's case it was a completely different thing. As written in character/rider description at the official site his fangire blood takes over and turns him into an aggressive person when he was bitten by Kivat and transforms into Kiva.

Once again though, it's up to the writers and the show itself to convey stuff, not just add details in bios never brought up in the real product.

For that you can say 95% of the Japanese shows are having the same problem.
A very common thing they do in their industry.

That doesn't make it okay though.

It is not ok, but what else you can do?
They are not going to change, and you are going to complain again and again.
Why not just make life a bit easy by understanding their way of doing things, step back and make use of google to fill in the blanks rather than hoping for a miracle to happen.

No, I'm going to blame the writer. Agito, Faiz, and Kiva are the only three Rider shows where it really doesn't feel like the dude in the suit and the dude who puts the belt on aren't the same characters. I'm sure there's a reason if you look into the apocrypha for all of those shows, but I'd rather see something like Ryuki, Den-O, or Fourze where the suit actor really captures the spirit of the character the face actor plays.

Honestly, its both the (action) director(s)' and the writer's fault.