Aside from the Zygons – and Queen Elizabeth the First, I suppose – Steven Moffat doesn’t seem to be dipping too far into the legacy and history of Doctor Who to populate his now-shooting 50th anniversary special.
The BBC’s latest official announcement is that Jemma Redgrave will be in the show. She played UNIT’s Kate Stewart, daughter of Brigadier Lethbridge-Stewart, in last year’s The Power of Three. That was her first on-show appearance – though I believe, and I was surprised to learn this, that Stewart first appeared in a semi-official home video series back in the mid-90s.
I dare say this casting announcement has been timed to happen now as Redgrave be on location shooting for the project tomorrow. As such, it’s a more or less equal assumption that any characters or cast members featured entirely in closed-stage shooting are likely to remain under wraps until much closer to air date, or maybe until the second they appear on screen.