The key to a great internal monologue in a novel is to make it authentic. Let the character's voice come through clearly. Also, use it at the right moments to add depth and insight. Don't overdo it or it might feel overwhelming to the reader.
For writing an impactful internal monologue in a novel, first, understand your character deeply. Know their personality, motivations, and fears. Then, show their thought process in a natural and flowing way. Avoid making it too complicated or forced.
Well, to write a good internal monologue, you need to make it genuine and reflective of the character's thoughts and emotions. Use simple and relatable language.