From the interviews after Rd 12. I realize they have computers besides them as they speak, but a couple times it looks like they are playing through the variations in their head. Very impressive, to me at least. Two specific instances are Ivanchuk early on in the video and then Kamsky/Aronian (about 5 minutes in). The latter was particularly interesting, when Kamsky gave a line that he thought was drawish, and Aronian interrupts him to give his defense to it (as he saw the possible variation during the game as well).
Finally, you can see how depressed Bacrot was after his loss. I can relate - and I’m glad I don’t have to answer questions from the press after my losses.
I found the video at ChessVibes; I think the following will work as the direct link to the video (full screen):