Puppetry and acting is a physical activity. The body is physical. The voice is physical. The breath is physical.
I struggled with this myself and I notice it in lots of performers. You say the lines over and over again and hope the character will magically appear. You think about the character as deeply as possible. You do lots of research. You try and feel like the character and hope that the physical character will somehow magically come from them. But it never does.
Because characters don't come from words, words come from characters. In the case of a play of course, the words have come from a writer, but even a writer might say that the words came from the characters. The job of the puppeteer, the actor, is to pretend to create the physical character to say the lines. So how?
What I do is to work out what the character is physically. How to "be" the character, without the words. For this I go to real life. This means finding someone that I can copy who I think is right for the character. Then I copy them. I get their posture, their walk, their mannerisms, their habits, their breath. From their breath comes their voice. And from that I start to feel what it feels like to be them. Then the idea is to get the character to do the text.
The idea is I never do the lines at all. I do the character and the character does the lines. Or in puppetry, I do the puppet and the puppet does the show.
The magic of acting is that if you imitate someone you feel like them. And somehow when you feel it you know that this is what it feels like to be them. It's kind of weird if you think about it, but it's definitely true.