Words or phrases in square brackets are thought/implied, but not said.
this is a really interesting thing for a writer to add. usually as an actor your intentions or internal monologue is completely up to you, but this writer gives moments that are thought or implied that the audience doesn't get to know about but the actor does. I'm assuming they had a really clear vision of how they wanted those moments to be and they didn't want to give their actors as much freedom in those moments.