Very well written. You did a great job at capturing Cosmo's emotions.

Your fic is very beautiful. I like how you intertwined the elements of the film so organically. I actually felt some sorta sympathy for Cosmo the first time I finished watching the movie for the first time. He was portrayed as being very talented, yet is somehow overshadowed by Don all the time; the seen where Don and Kathy kissed in celebration of Cosmo's "Dancing Cavalier" idea pretty much summed it all up. I love how you pointed it out.
Your fic actually wants to make me watch the movie again.
Keep up the good work!
-Love, Dub Video Dub