 SPOILER: According to Joss Whedon (qv) in his commentary, the most  difficult scene to get the sound right was the scene between Simon  and River at the end, after Simon had been shot. Whedon says that  the bond formed between Sean Maher (I) (qv) and Summer Glau (qv)  over the course of the shooting of Firefly was so strong, and Seans  acting so good, that Summer kept bursting into tears whenever they  attempted to shoot the scene.