Before we start. Huge spoiler warning here. This goes into the very end of the film, so if you don't want to know, get out of dodge, now!
Now they are gone, let's get into it. It's been revealed that by director Scott Derrickson to IGN, that Benedict had another secret role in Doctor Strange that no one know about. He is at least a part of bringing Dormammu to life in the film, who shows up as the big bad at the end of the film. He appears as a big floating head, and the director has confirmed that the facial capture was provided by Cumberbatch. He wouldn't give away the name of the voice actor, but talked about it conceptually, saying:
It's an interesting idea, because I did fancy the idea that as this other-dimensional being he's not moving through the Dark Dimension, with a countenance, with eyes and a face, eyes and a mouth, and a human visage – why would he? He takes that form on to communicate with Strange."
The more I thought about it the more I liked the idea, because no one understood Dormammu better than Benedict did. I also wrote that role to be a kind of ultra-inflated version of Strange. He is an ego run amok; he is this cosmic conqueror where everything, where literally everything in the multi-verse is about him.There's something interesting about this confrontation of this little, tiny guy who has this power of time and this monstrous conqueror who is trapped by a clever gambit. There's something about that worked well, and I didn't think anybody to interact with Benedict than he, himself.
This is a pretty cool little nod. While I am kind of sad we didn't see a flaming head, human sized version of the character, I like the ideas behind this.