Check out the set photo of a very awkward hug between the latest Doctor, Peter Capaldi, and his companion Clara (via Ryan Farrell). What's going on in this scene? We have the first hint, and it involves a huge, spoilery cameo by the absolute last person you'd expect.
One of Bleeding Cool's readers was on the scene for the filming of this sequence, and provided a pretty full description of what they believe is happening here. The scene begins with Clara emerging from the TARDIS talking on her mobile phone, looking "intense and emotional" and not speaking — just listening. Whatever she hears, it hits her hard, and she slumps against a nearby storefront.
And it turns out she's hearing a voicemail from the previous Doctor, played by Matt Smith.
Capaldi's Doctor emerges from the TARDIS and asks Clara if that was the Doctor on the phone. Then Capaldi insists that he is the Doctor, he's 2000 years old, and he's standing right here, in front of her. Clara walks up to him and looks him in the eyes, searching for traces of the man she knew. The Doctor looks down at her, somewhat bemused, and then she throws her arms around him while he holds his arms out awkwardly — as you can see in the above picture.
Then Clara clicks back into her being her usual bouncy self, asking the Doctor where they are, and he replies Glasgow.
Apparently Capaldi was heard on the set several times, asking what "Matt's lines" would be on the phone, which makes it seem as though we'll actually hear a message from the departed Doctor in Capaldi's first outing. As usual, take this with a grain of salt until the episode actually airs. [Bleeding Cool]