Doctor Who: 14 Cool Details In Steven Moffat & David Tennant's Silence In The Library Commentary
11. There Was Almost A Longer Scene Set Inside The TARDIS
One other idea that Moffat had for extending the episode was to include a lengthy scene set inside the TARDIS, but in the end, this never came to pass.
The TARDIS isn't really in this two-parter very much, which is presumably why Moffat felt like there was room for more TARDIS-oriented material.
So what would this extra TARDIS scene have been? Well, Moffat states that he wrote "two pages of absolute waffle" in which the Doctor and Donna talk about a beach while stood around the console. In the final cut of the episode, Donna says that she and the Doctor were planning to "hit the beach" prior to arriving in the library, so this extra scene would've expanded on that a bit more.
Amusingly, Moffat actually used this "adding a TARDIS scene to flesh out an episode" trick when he became showrunner. As we already mentioned, The Time Of Angels came in too short, so Moffat scripted in that early TARDIS scene involving the Doctor, Amy and River, to make the episode a suitable length.
Shooting in the TARDIS doesn't require as much money as a location shoot, so it's a quick and easy way to add more scenes to an episode when necessary.