David Farmer did an AMA on Facebook recently where he talked about some of this:
David Farmer: It helps to start with a great performance which Benedict gave us in droves. His voice is arguably the best I've been able to work with. The key was the right amount of pitching, with other low end enhancers, for the most part. But then there was the reverbs to make it sound like it excited the space, as well as vocoding alligator growls to add an extra layer of girth underneath. There were quite a few processes, some added quite subtley, but in the end the sum of the parts created something I hope sounds natural, but yet authentic.
David Farmer: The FIRST thing I wanted was for the beast part of Smaug to be an alligator. I love the sound of an alligator growl, but have never heard it featured anywhere for a film beast. I didn't want him to just be a lion or tiger, or some pitched version of those and the alligator fit the bill. I'd heard alligators before but never had the chance to record them. I want to thanks Colin Hart for sharing his knowledge of Alligators which led me to get several days worth of recordings.