I googled the expression “nervous as a cat” and it comes from the idea that a cat moves very quickly and suddenly. It is used when a person is uneasy or overly alert. I don’t think Felix is uneasy in the least. But he is very alert. He can spring into action from complete stillness, zoom down the hall, bounce off the wall at the far end, return to hit the back of the sofa, and land on a chair—all in a moment. He has a lot of fun doing this.