The actress told DS that the two-parter resolves a lot of unfinished business from the past eleven episodes, especially for her character Amy Pond.
Gillan said: "There may be cracks in time and space involved. There's a lot of loose ends being tied up and things being explained and revelations."
Matt Smith added: "Let's just say that The Doctor's up against it. He's as up against it as I've ever seen. So whether that be a Dalek, or a Cyberman, or a Silurian. Who knows?
"I think they're certainly two of our most ambitious episodes - they're certainly the most accomplished. I think Steven [Moffat's] writing has somehow got better... It's everything that Doctor Who should be and could be when it's firing on all cylinders."
Gillan also officially confirmed that her character will be "coming back" to "do it all over again" when the show returns in 2011.
> Click here to watch our interview with Karen and Matt