It was very different from what I usually write and very challenging, which I think is part of the point of the exercise. We're not allowed to say anything that would reveal the fandom or anything specific about the request and I will honor that, but I will say that it's fanfic based on a novel. I was really struck by how much more constraining that is than fanfic whose source text is movies or comic books.
With movie and comic fanfic (and, I think, with tv as well, although I haven't written any) the author is bound to write a story that fits with canon and to make the characters consistent with how they are portrayed in canon. So you need to know what happened to them, you need to know their speech patterns and personality characteristics and so on. But beyond situation, plot and character, you have a lot of leeway in style and tone.
With a novel, I think you need to match the style and tone of the original. Or, at least, I felt bound to do so. Now that I think of it, a lot Harry Potter fanfic doesn't really sound like Rowling, but I digress. Anyway, I felt I wanted to sound like it could truly be another chapter of the novel, so I needed to write like the original author did and I found that very challenging.
The story also takes place in a different place and time from what I'm accustomed to writing, and the characters are very different from what I usually write, too. In addition, since my story concerned activities alluded to but not discussed directly in the original, I needed to research specialized terms the characters would have used in that time and place.
So, all told a really challenging assignment, but a fun one. I'm pleased with how it came out and pleased to be done in time. Now I'll go back to the X-Men series I was working on.