Doctor Who: 14 Cool Details In Steven Moffat & David Tennant's Silence In The Library Commentary
10. Moffat Had To "Fight" For One Particular Effects Shot
Silence In The Library marks the Whoniverse debut of Alex Kingston as River Song, with the character arriving in the library as part of an expedition.
Just before she greets the Doctor with a signature "hello sweetie", the visor of her helmet sort of "de-fogs", and according to Gardner, Moffat had to "fight" to get this effect included in the episode.
The reason for this is because the episode already had more effects shots than the production could afford, and Gardner felt like this small one could be chopped to save a bit of money.
Clearly though, she changed her mind in the end, and that's because Moffat persuaded her to keep it in.
During the commentary, he states that the Doctor coming face-to-face with someone important (like River) is a big moment, and the fog effect makes their first encounter feel more "special", for lack of a better term.