How much impact does the Doctor changing have on other people?

Question

It is a well known fact that the doctor changes forms semi-frequently. However, all flashbacks or visions of "future" events, seems to always portray the current doctor in them (which I can understand from a casting point of view). However, some people seem to be able to recognize the doctor after a change, some seem to not notice he has changed, and some do not recognize him until they see something that helps them realize he is the Doctor.

Is there some explanation for why the current doctor seems like the only doctor? How recognizable is the Doctor after he has transformed? Do some people have an uncanny ability to simply know who the Doctor is?

Answer 1

A lot of it depends on the writer, and the story in question. A majority of Doctor Who stories take place in the Doctor's "recent present", so flashbacks or future visions don't go too far into the past or future of the Doctor's timeline. There's also generally less references to past Doctors in the revived series, most likely due to confusion for viewers. That said, there are definitely points where past Doctors are referenced. In terms of new show, the 11th Doctor's first appearance (The Eleventh Hour) directly shows clips of past Doctors. In the older serials, the Third Doctor's first encounter with the Daleks (Day of the Daleks) resulted in them needing to scan his mind to find out that he is The Doctor that they remembered.

Judging by the reactions of characters who meet a "new" Doctor, most people don't have the ability to recognize the Doctor once he regenerates. However, they'll often "recognize" The Doctor once he introduces himself, and act as old friends, such as the Brigadier, or old enemies, such as the Daleks.

That said, there's a few people who probably do have the ability to know who the Doctor is, without the process of a brain scan or an introduction. These are mostly the "super powerful" aliens, including other Timelords, the White and Black Guardians, and the Eternals. It generally seems that races that are as powerful or more powerful than the Timelords know who the Doctor is even throughout his regenerations.

Answer 2

It's not the case that people either don't notice the Doctor's regeneration, or automatically recognise his new form. In fact, the reverse is normally true: the companions don't recognise the Doctor, and take some time to accept that the new face is really him. Even in the recent series, Rose took a whole episode to accept that David Tennant was really the Doctor - but this goes right back to the early days, for example in Spearhead from Space when the Brigadier didn't recognise Jon Pertwee.

As far as I can tell, the only people who have shown an ability to recognise the Doctor across regenerations are other Timelords, particularly the Master but also including the various people we encounter on Gallifrey in those stories (mainly Fourth/Fifth/Sixth Doctors) that went there.