I think it is Haley Bennet, who was just selected to be in Malick's next film with Christian Bale hence they are at the festival together. She was most recently in the Haunting of Molly Hartley.
Never mind Bennett and Bale. That's the legendary Terrence f*ing Malick you got there!!! One of the best photos ever taken of him and (until recently) very rarely photographed because he doesn't do interviews or like his photograph taken. Good one!:)
Here's the video that this screenshot was taken from: http://nworbleahcim.blogspot.com/2011/09/haley-bennet-terrence-malick-filming-at.html
ACL Fest 2013 Lineup Announcement Video from Moontower VFX on Vimeo.
ACL Mike is in no way affiliated with Austin City Limits, ACL, ACLFest etc. ACL Mike just likes going to ACL Fest and talking about ACL Fest - thus the nickname.