Pixar's new animation system allows animators to visualise their characters with all the important detail visible. Like hair!
Pixar's Monsters Inc was a groundbreaking film in itself, but technically, it was one of the first times we saw realistic looking hair behaviour in an animated production. Hair is a menace for animators, because each hair can act independently of all the others, needing intense modelling and computing to produce the final result.
These films are not rendered in real-time like computer games. The resolution and detail is so high that until very recently it could take hours per frame for the final output. Now? Well, we're talking about on-screen visualisation here, not the final cinematic output, but nevertheless, the new Presto system allows real-time access to every articulation point and behaviour of the characters - as well as rendering that hair!
Here's a video of Pixar talking in detail about Presto, which is fascinating in itself, but also gives us a glimpse of how the characters are made and animated for the actual movie.