Possible and mild spoilers here.
The footage shows the 12th Doctor in… um, a nightgown? He jokes; “This is not the Doctor’s costume”.
It’s good to see Capaldi being happy to spend time with fans rather than get grumpy about them filming. That’s especially true since the production team did have to ask the fans to watch their camera flashes.
File this one under “Watch it before it’s YouTube Yanked” as it seems possible the BBC will want it pulled.