Well, internal monologue is important as it helps reveal a character's hidden motivations and inner conflicts. It also adds depth to the story and makes the characters more three-dimensional. Plus, it can enhance the reader's emotional connection with the narrative.
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.
Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.
Misa would likely be excited. She is a big fan of Light and the whole 'Kira' concept. In fanfiction, if there are scenarios where she has more power or a closer relationship with Light, she would be all for it. However, she might also get jealous if there are other female characters getting too close to Light in the fanfictions.