For short stories or a bit longer ones, I write a scene or a piece of a scene from the viewpoints of each character; then I choose the one that sounds the most authentic to me, but I don't know if that would work for a novel since it is longer and more complicated. I never tried that for a novel.

Maybe we do the same or a similar thing with a scene from the beginning, a scene from the middle and another from the end, too? That might work if we are sure of the ending.

In my novel's case, I definitely want my main character to be heard not only with what she say but what she thinks and feels. So it will be her voice as the dominant one, anyway.