Interior monologue is a handy dandy tool.
Here are my rules for interior monologue:
- Use it to disclose information that would be difficult to disclose with dialogue.
- Use it to develop character: show the character's traits and/or emotions
- Don't overuse it. During revision, look for long portions of interior monologue and cut, cut, cut.
- Use it to limit the use of speaker (or thought) attributions such as she thought or he wondered. For instance, try converting a sentence with a speaker [thought] attribution to a question. Instead of, "He wondered why he always ended up lying" try: Why did he always end up lying? You don't even need to add "he wondered" if you are 100% inside the character's head. The reader will get it.