So the goal would be for the writer to not only nail speech ticks, rhythm and dialect, but to also perhaps express something of the essential nature of the character in the line. Because this is how I feel about Whedon's dialogue--he manages to fit so much into so few words, telling you about the plot and the characters and humor/drama all wrapped up into a line. So that's the challenge.
Game sounds fun? People feel free to kick it off in the comments if you want to play. I'll be baaaaaaack. :)
ETA: And I think it'd be great if people explained why they think it's whoever when guessing and perhaps the writer will share why they wrote that line and how it's in-character (dialect, word choice, etc).