I'm not sure specifically as there could be many different interpretations of a 'before the fawn fanfic'. It could be about events that happen prior to the appearance of a fawn in a particular fictional world. Maybe it details the backstory of a character who later encounters a fawn, or it could be about the setting and circumstances that lead up to a fawn - related event in the story.
I'm not sure if it's popular. There are so many fanfics out there, and without more information or a broader fan community's input, it's hard to determine its popularity.
Since I don't know the details of the 'Atticus Fawn' novel, it's difficult to say what makes it special. But perhaps its unique combination of characters, like the eponymous Atticus and the fawn, could be a factor.
The 'Atticus Fawn' novel is not a widely known one to me. However, if it's a fictional novel, it might revolve around a character named Atticus and perhaps a fawn could be a central element in the story, like it could be a symbol of innocence or part of a journey that Atticus undertakes. Without reading it, it's hard to say for sure.
I'm not sure specifically as there could be many interpretations of a 'jily' (James and Lily from Harry Potter, I assume) fanfic named 'before the fawn'. It might be about their relationship before a significant event symbolized by the fawn, perhaps a magical encounter or a turning point in their story.
Maybe Harry somehow gets transformed into a fawn or has a connection with a fawn. It could be a magical adventure where he has to navigate the world as a fawn or protect the fawn from some sort of danger.
There's no way to know precisely without reading it. However, it could be that there are human characters if the fanfic is set in a world where humans interact with the fawn. For example, a young child who is curious about the forest and might soon discover the fawn. Or perhaps there are magical creatures other than the fawn that are central to the story before the fawn comes into the picture, like wise old owls or mischievous squirrels.
You could try searching on popular fanfic websites such as Archive of Our Own (AO3). It has a large collection of fanfictions, and if the 'before the fawn jily fanfic' is out there, there's a good chance it might be on AO3.