Q: I didn't get what does he mean by
"taking over" and "roundness and life".

Jacob was my first experience with a character taking over - a minor character developing such roundness and life that I couldn't keep him locked inside a tiny role. とはどういう意味ですか?
A: The writer is describing the characters he creates as if they develop a life of their own, without his conscious control. He talks about that as the character taking over (gaining control) of the process.

If a person (or a fictional character) is well-rounded, that means that he is well developed in many different ways, not just in one narrow domain. Minor characters are generally not well rounded, but major characters are, because well-rounded characters are much more interesting. Here he's using "roundness" to mean something like "well-roundedness". He's saying that he finds his character developing such a complex personality (as if by itself without conscious input from him) that it would be a shame to keep the character in a minor role.


Q: The roundness of her lip, shoulder, arm, and fingers are symbolizing the tenderness of women. この表現は自然ですか?
A: The curve of her lip, shoulders , arms , and fingers are symbolizing Herself.