Warning: spoilers and speculation ahead.
After yesterday's brief stop-over in the present day, they were back to shooting crowd scenes in pseudo-Victorian London again, presumably still for episode 1.
According to Twitter user RyanFarrr, most of the action today involve our mysterious top-hatted guest star (who has a cybernetic eye-piece over his left eye you can't see in the photo) walking down the street "with stuff on his face" followed by a large crowd as gas lamps are being lit. He then enters a house and possibly looks out the window, with period extras "walking past and glancing, then looking away nervously." As the same actor has been seen entering and exiting the set on multiple shoot days, we can likely assume he's one of the episode's major characters, and probably a reasonably well known actor, whoever he turns out to be.
You'll notice in the above photo his hat appears to have green CGI markers stuck to it, and the big "silver ball on a stick" CGI marker also made an appearance shortly after Mr. Top Hat's street scene, so you can make of that what you will. Some other folks noted that in addition to the eye piece it looked like his nose was partially prosthetic, which if true, would be a good indicator that we are supposed to be seeing his face, rather than simply being a CGI stand-in for somebody like Dr. Simeon/Mr. G. Intelligence.
Also, for any of you who missed yesterday's filming, here's a nice video (don't bother with the sound) showing not only the filming of the street scene, but a few bits of their video playback:
The major spoiler from this scene comes from a set reporter who says the call that Clara receives on her mobile is from The Doctor. The 11th Doctor. Calling from Trenzalor, presumably during the gap in The Time of the Doctor between 11's big Dalek-blasting clock tower boom, and when Clara enters the TARDIS, placing the dangling receiver of the TARDIS phone back on it's cradle as she does.
Thanks to a rather good shot of the clapperboard, we can tell that this is a scene from very late in the episode, and probably the final scene itself, since after 12 says "It's me. I'm standing right here" and the awkward hug, they walk off past the TARDIS.
We also have a rather good shot of the gobo (the light up TARDIS interior thingee they stick inside the police box) which appears to be the same one they used in series 7, but it's a bit hard to tell. At least one person on Gallifrey Base insists we're getting a tweaked console room, and the demo version of the 12th Doctor Character Building playset from this year's London ToyFair looks like this:
Whereas the unreleased 11th Doctor console room set from last year looked like this:
Admittedly, it's not like these are the most detailed toys ever, and the general architecture seems more or less the same. The major difference is that the console in the new version appears to be dark grey or black, with a dark blue time rotor. (though it may just be an attempt to match the battleship grey coloring of the current console.)