Doctor Who: 12 Cool Details Revealed In Steven Moffat's Blink Commentary
7. David Tennant Recorded His Lines By Himself, To Make Them Sound More Authentic
Once again, the majority of Tennant's screen time in Blink is limited to the DVD easter egg. Within the context of the episode, the Doctor is sat in a room by himself (with Martha nearby), speaking into a camera, recording a message that will slowly make its way into the hands of Sally.
Basically, he's trying to have a conversation with Sally, without her being in front of him. All together now: wibbly-wobbly, timey-wimey.
And so, to make the Doctor's half of the conversation feel as authentic as possible, it was filmed in an authentic manner: David Tennant sat in a room and spoke into a camera, all by himself, with no Carey Mulligan to bounce off. Poor fella.