This video is mainly just David Tennant's clips (either David speaking, others talking about him, or context) from a documentary about the making of The Quatermass Experiment from 2005.
In this one-off drama David played Dr Gordon Briscoe. Among those in other clips are Mark Gatiss (who played Paterson) and Andy Tiernan (who played Victor Carroon).
Mark and Andy talk about being there when David got the news that he'd been chosen to play the Tenth Doctor.
Many thanks to Just Tennant for uploading this video.
Apologies if this video does not play in your country.