Doctor Who: 14 Cool Details In Steven Moffat & David Tennant's Silence In The Library Commentary

Since the bloodthirsty Vashta Nerada resemble ordinary shadows, the crew that worked on the episode had to constantly be mindful of any shadows that the equipment and the furniture (and themselves) were casting.

And this posed several production challenges. During the commentary, Gardner mentions that the library location was "difficult" for sound, because it was tough to figure out where to place the boom mic without it casting a shadow. The main room in the episode was also very echo-y, which didn't help matters.

In addition, the lighting was tricky to get right too. That main room was also very dark, and so, according to Tennant, a "huge white light" was put up. Unfortunately though, this light was so massive that it had to be cooled by a noisy fan at all times, which drowned out any dialogue that was spoken too softly.