With tfa this is tricky, but repeated viewing/consumption of the source material. You reach a point where you can hear the characters speak in your head.

For a character like Phasma, who doesn't have a ton of screen time, I would look to other sources of inspiration. Perhaps the Clone Wars series, which featuresa lot of clone trooper interaction and culture that you could build on. A lot of people are revisiting Timothy Zahns Thrawn trilogy for inspiration and references to use when writing First Order characters.

Phasma might be rather easy to work with since she doesn't have much screen time- as lkng as you work within the framework we have for the First Order, you could characterize her however you like.