This alerts us to a new event and a new person in the story. (See: [[rc://en/ta/man/jit/writing-newevent]] and [[rc://en/ta/man/jit/writing-participants]])
This is an idiom that probably here means "began to act." Your language may have a different way of showing that the expert in the law had been present, listening to Jesus, and was now beginning to act. He was not necessarily sitting before he "stood up" and began to speak. (See: [[rc://en/ta/man/jit/figs-idiom]])