Automated voice casting gets you a reasonable starting point, but the difference between a serviceable read and one that actually serves your characters usually comes down to a few deliberate adjustments. Here's how to think about voice selection with more intention.
Start with what actually distinguishes your characters
Before adjusting any voice, ask what's supposed to set each character apart from the others in a given scene — age, formality, energy level, warmth versus coldness. The goal isn't necessarily to find a voice that matches how you personally imagined the character; it's to find a voice that's clearly distinct from every other speaker in that same scene, so a listener never has to guess who's talking.
Prioritize contrast within a scene over "perfect" individual fit
Two characters can each have a voice that sounds fine in isolation, but if they're too similar in tone and pacing, a listener will lose track of who's speaking the moment the scene picks up pace. When reviewing voice assignments, listen to characters back-to-back rather than one at a time — that's the real test of whether the casting works.
Match tone to function, not just personality
A narrator voice, for instance, generally benefits from steadiness and clarity over strong personality — its job is to carry the listener through the story, not compete for attention with the characters. Characters with big emotional swings benefit from voices with more natural variability, since a very flat, steady voice can undercut an intense line.
Use previews before committing
Always preview a voice on an actual line from your script rather than judging it from a generic sample — the same voice can land very differently on a whispered line versus a shouted one. A quick preview against your real content catches mismatches that a generic sample wouldn't reveal.
Revisit casting after hearing a full scene
Sometimes a voice that reads fine on a single line reveals a problem once you hear it interact with another character across a full exchange — pacing that clashes, or a tone that doesn't sustain well over several lines. It's worth listening to the complete scene once before finalizing voice choices, rather than approving each character's voice in isolation.