It's all done in 2d. you render the elements separately and then use shake or after effects etc to "reveal" them as required. You can't animate the boolean tools or anything like that. And you wouldn't want to.
It's much better to do less in the render and more in post as you have more control overthings there and you don't need to keep re-rendering.
Alan
Technical Director - Framestore
Currently working on: Your Highness
IMDB