A movie fanfic is basically a story written by fans based on a movie. It can explore alternate scenarios, add new characters, or give a different ending.
Well, you could start by imagining unique and charming encounters. Maybe your character stumbles upon the movie star at a coffee shop or a charity event.
Well, if you want to write an 'A Star Is Born' fanfic, you need to immerse yourself in the world of the movie. Read other fanfics to get an idea of what has been done before and what you can do differently. After that, come up with a unique angle. It could be a time - travel story where a character goes back in time to change a crucial event in the movie. Or it could be a story that focuses on the making of one of their songs. Then, when you start writing, use vivid descriptions to bring the story to life. Describe the settings, like the smoky clubs or the big concert arenas, in detail.
In '10 Cloverfield Lane fanfic', it differs from the original movie in multiple ways. The movie has a linear narrative for the most part, but fanfic can be non - linear. It can jump back and forth in time, showing events before the characters entered the bunker or imagining future scenarios. The fanfic can also change the nature of the threats. In the movie, the threats are mainly the alien - like creatures and the psychological pressure in the bunker. Fanfic could introduce other threats like a virus spreading within the bunker or a traitor among the survivors. Moreover, the fanfic can provide more backstory for the characters. We might learn more about Howard's past, his family, or why he built the bunker in the first place, which was not fully explored in the movie.
The most important element is probably the connection to the original Harry Potter world. The fanfic might introduce new concepts, but it should still feel like it belongs to the same universe. The casting of the main characters also matters a great deal. They should look and act the part. Another aspect is the pacing. In a movie, the story needs to move along at a good speed. If the fanfic is slow - paced, it might need some adjustments to keep the audience engaged.
Maybe a fanfic exploring the characters' lives after the ending of the movie. It could focus on how they cope with the ongoing challenges and build a new normal.