Warning: spoilers if you haven't seen episode 8 of the current season, "Silence in the LIbrary".
From Doctor Who Confidential, Steven Moffat explains his thought processes:
Also worthy of note, which I missed from this clip, someone (I think the producer) saying something along the lines of "The brilliant thing about Steven is that he writes this invisible monster that lives in the shadows, and then somehow pulls out a lurching skull-headed zombie in a space suit".
