A friend told me about this story, but she can't find it again and neither can I.
It may be gen or Arthur/Ariadne.
The plot of the movie is basically therapy for Cobb and is in his head, and Ariadne is some sort of therapist or involved with the therapy in some way.
I'm not sure if the rest of the team is also involved, or if they're projections.
Found by
cunning_croft. It's
Thimblerig by
cupiscent Thanks also to
tialys, who reminded me of a similar fic (and it's the one where it all happens in Cobb's head),
Amazing Grace by
airgiodslv