I think it's difficult to keep them apart when you play quite a few, cause there's also the "narrating" voice (yours) that's supraimposed, so - don't... blame yourself 100% T_T

In my experience, at least: it really helps to introduce verbal ticks, or phrases you associate with the character, or even a voice in your head.

Voices....


...in....


....your head.....





.................................rightee-oh.