Turns out the Warner Bros. has not released the official synopsis for The Hobbit: The Desolation of Smaug until now. Normally these get the released around the same time they first start teasing the movie so not sure why the delay.

Of note is the Cate Blanchett, Christopher Lee, and Hugo Weaving are not mentioned. This could mean they have no role or the role is much smaller than even in An Unexpected Journey. I suspect they are not in the movie as there was no means to insert them into the middle story events. All the characters are still on the same journey they were when left Rivendell in Journey so really no reason to swing back to those three in an organic way. Below is the synopsis.
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The Hobbit: The Desolation of Smaug Official Synopsis. There are any The Hobbit: The Desolation of Smaug Official Synopsis in here.