According to my experience, you CAN'T create a story, any story, without talking it through, whether it's out loud or in your head. At some point, you open your mouth or your mind, and you visualize what someone will say or do.
See, some people can just think these thoughts and not actually "talk". Me, I most certainly have to talk. I can't visualize until I actually HEAR what my characters are saying, and how. if they have accents especially, I need to HEAR it. Even if I'm writing a scene with no dialogue at all, I have to hear what is going on to help me work it out.
Also, I have a terrible stammer since I was two years old. It's hard for me to speak to others so me talking to myself while I work out a script or comic panel, or book, or story, is my way to practice. I don't stutter if I talk when no one's around to hear. 