Benedict Cumberbatch is doing the motion capture performance Smaug and voicing for the Necromancer (aka Sauron).
"I'm in it and I’m playing a dragon and I’m also playing the Necromancer. And I’m not – repeat not – doing the voiceover,” 
"I am playing him (Smaug). I am the voice but I am playing the moving, physical dragon. It’s very exciting.” 
Following in Andy Serkis's footsteps, Cumberbatch will be creating the overall performance and physicality for the dragon Smaug as well as voicing the Necromancer. Actor and stuntman Sala Baker played Sauron in The Lord of the Rings trilogy.
As Sauron is posing as the Necromancer, the evil sorcerer and concealing his true identity, hopefully his depiction in The Hobbit films will differ from his appearance in the prologue of The Fellowship of the Ring (2001).
Cumberbatch was quoted by The Guardian as saying:
"I'm playing Smaug through motion-capture and voicing the Necromancer, which is a character in the Five Legions War or something which I'm meant to understand," and "He's not actually in the original Hobbit. It's something [Peter Jackson]'s taken from Lord of the Rings that he wants to put in there." 
"Smaug destroys Lake Town", John Howe
The Guardian speculated from Cumberbatch's comment that the Necromancer may be playing a much more significant role in The Hobbit films, than he did in the book. Hopefully, Peter Jackson and the film makers stay true to the book, as the Necromancer does not appear at the Battle of Five Armies, which Cumberbatch mistakenly referred to as "Five legions War".