At the Los Angeles Times, Domhnall Gleeson talks about why the character of General Hux appealed to him:
I liked him because he allowed me to explore the notion of authority — he’s someone who enjoys authority and holds onto it at all costs. The costume was brilliant and added a whole other layer for me. We talked about the idea that he’s an obsessive and probably doesn’t sleep much.
So he was pale with dark eyes. We wanted him to look like somebody who was dead set on one thing.
Meanwhile, Gwendoline Christie discusses Captain Phasma’s lack of face time with Entertainment Weekly:
I thought it was a really interesting opportunity to play a female character where we formed an opinion of her based on her actions rather than the way she has been made flesh. And that concept within a Star Wars movie, a mainstream phenomenon, was very modern and interesting and exciting.
Both characters are confirmed to appear in Episode VIII.