This might seem a little odd, but usually, I associate each character I create with a certain spoken voice - usually an actor/voice actor from a game, movie, anime - whatever's the last thing I watched that left enough of an impression on me that it's the first thing I think of when creating a character, and thus "cast" the voice/actor I'm thinking of as the character. At that point, I have not only a defined mental image for the character - how they look and how they move - but also a defined voice in my head that I can imagine delivering their lines. Oftentimes, just to make sure it sticks, I'll do my best impression of that particular voice and act out their lines to get a particular feel for how they sound and the way they'd respond to given comments. At a certain point, I just stop thinking about it, and let the scene play out with the "voice" speaking each line as it occurs to me, such that it's more like watching a pre-scripted episode of a TV show or a video game cutscene than coming up with something in pieces.