The video panel from the 2012 San Diego Comic-Con was posted on youtube. One word River=Summer Glau.
Here is a short clip of Summer's part:
Summer explains the key of playing River. Joss whedon talks about casting Summer and why she was perfect for the role.
Here are videos from a mystery panel live from nerd HQ:
Screencaptures from the panels could be found here Interviews > 2012 >