Then maybe at first she was playing the woman in the book, and then they reshot with Isla Fischer?

You could be correct, but I doubt it.
Here it is:
Nocturnal Animals(2016)
Amy Adams revealed on NBC's "Today" that the 'hillbilly' night shoot in California's Mojave Desert lasted about 10 days and was such a grueling and emotionally-draining experience that she was thoroughly relieved when the film wrapped.
http://www.imdb.com/board/14550098/trivia?item=tr3184793

Maybe she means her reactions to what happens in the desert.

She is best friends with
Fisherbut doubtful she was on set for much of the time for the desert scenes, which she doesn't
(if at all) because, in addition to "Nocturnal Animals", she was filming "Arrival" and "Batman v Superman: Dawn of Justice"
Plus, the HBO' series "Sharp Objects"
ALL in 2016

she was probably on set though. or just incorrect trivia written by someone.

*Gingers!!!
Damn autocorrect malfunctions.

I guess all fingers look alike to him. Racist!

In case you may delete it, I'm gonna quote it.
Listen I don't mean to be rude but are you serious ?
35% of the movie was her with jake and their daughter in the desert - just cause there is a road there doesn't mean it's not the desert - are you ok over there ?
You are funny as hell. It's Isla Fisher, dude.

Yes the OP is serious Amy Adams isn't in the desert scenes you are mistaken for Isla Fisher in the scenes of Edward Sheffield's book to Susan not Amy Adams

Listen I don't mean to be rude but are you serious ?
35% of the movie was her with jake and their daughter in the desert - just cause there is a road there doesn't mean it's not the desert - are you ok over there ?