In the movie, Death on the Nile, the movie starts off with the wedding of a couple, them being happy, and everyone having a good time. From the title of the movie alone, I knew that there was going to be some sort of death and mystery involved, but starting off with such a happy scene made me think that the murder was going to happen in an different setting with different people. Then, about 15 minutes later in the movie, the wife of the newlywed couple was murdered. The attempt at mixing romance with a murder mystery was successful in keeping my attention because I didn't expect the wife to be the one getting murdered. I think the fact that it was so unexpected and sad made this attempt work. Clearly when writing the script for this movie, the screenwriter knew and understood both forms well enough to make this switch work effectively. In summary, I thought that the mix of forms in the movie, Death on the Nile, was effective, and I think it is something people can use to keep their audience's interest if done correctly.
Omg I LOVE your example. I totally agree that mixing the romance with the murder mystery was successful and was still respectful to the normal forms of movies.