The Curator

The curator of the National Gallery in London was an elderly individual who walked with a cane, and who had insight into a particular Time Lord painting.

After the Eleventh Doctor remembered his face (which resembled that of the Fourth Doctor), the curator told him that "in years to come, you might find yourself… revisiting a few. But just the old favourites, eh?"

He spoke with the Eleventh Doctor and told him that the name of the picture was neither No More nor Gallifrey Falls but in fact Gallifrey Falls No More, and pointed the Doctor in the direction of a search for Gallifrey, telling him he had "a lot to do".

The curator then suggested that he had perhaps been the Doctor once, or that the Eleventh Doctor had once been him. (TV: The Day of the Doctor)

Behind the scenes
Conservateur (The Day of the Doctor)
 * The script never directly states that this character is the Doctor, but implies that he may be a future incarnation who has adopted the same form as his fourth, or is in fact, said previous incarnation.
 * Tom Baker is not separately credited for this performance, receiving only a credit for playing "the Doctor" (as did all the other past Doctor actors, who appeared in the episode through the use of archive footage).
 * Early in the episode Queen Elizabeth I's letter appoints the Doctor as "curator of the Under-Gallery."
 * Tom Baker played a similar nameless character, with the appearance of an older Fourth Doctor, in a series of idents for the 1999 Doctor Who Night. Like the Curator, he is never stated to be the Doctor and refers to him in the third-person, but is shown in the TARDIS and says that he's "called Paul McGann in this one" when introducing the TV movie.