"It's a different kind of anniversary, and the spine of that is David and Matt." Steven Moffat On Doctor Who's 50th Anniversary Special

Steven Moffat has been talking to Entertainment Weekly about the eagerly awaited Doctor Who 50th Anniversary Special and has stressed that the episode very much revolves around David Tennant and Matt Smith's Doctors.
"People say, 'Are you going to do the eleven Doctors?', We can't do eleven Doctors! Three of them are dead!"
And although the episode will see fan favourites David Tennant and Billie Piper retun Moffat says it's not a big reunion style episode:
"What we're not doing is the traditional massive reunion. It's a different kind of anniversary, and the spine of that is David and Matt."