The facial mocap system we used for the short thanos scene in the first avengers was an Xbox Kinect and some open source libraries.
Something like the kinect can be very powerful for facial tracking. But monocular optical techniques tend to give poor body capture results.
The tracking data was not used for the final rendering, but was a basis for the animation team to work from.
You can definitely see how much it has improved when comparing that scene to the more recent iterations thanos.
The Xbox Kinect was such a versatile, and well-built device. Its a shame that its been discontinued and didn't find much use in games.