Stoked for the movie! How'd you decide on the overall design? He seems more like a traditional looking dragon so I'm guessing you didn't feel like taking much influence from the animated version of the The Hobbit?
As I recall, the book mentioned Smaug having a red glow when Bilbo was approaching his lair but whether that's from some sort of bioluminescence or just the light of his scales reflecting off all the treasure I wasn't really sure—either way I made his eyes glow in lieu of that. I tried to incorporate that long sinewy shape Smaug had from the drawing of him on Thror's map. At the same time I didn't want him to look too much like an eastern style dragon and his pride and haughtiness reminded me of a cat so his limbs are somewhat tiger-like. I had seen Tolkien's illustration of him and tried keeping the body shape similar but his head in that drawing didn't convey the measured danger he gave off in the novel. In fact most of the artists' take on Smaug I had seen didn't really seem to give him much personality except for Justin Gerard's awesome piece. I read up on how dragons worked in Tolkien's legendarium and it seems they were evil spirits made form by Morgoth, so I gave my Smaug a sort of devilish expression. The glowing eyes and mouth are similar to how the balrog design in the film looks so I'm glad to have made some sort of visual consistency between them. Both seem to be creatures of fire and shadow after all.
Anyways I didn't mean to write a whole block on my design thinking and whether or not I executed it well depends on the audience. Still, thanks for the question. I'm looking forward to see how the film handles Smaug.