Characters in TV shows are usually established within their first episode, or perhaps even within a few minutes of their first screen appearance, so when it comes time to write them out, it’s natural for the public to be a little wary of the new character that the takes place of someone familiar.
In this respect the Doctor is something of an oddity in TV land, as a character who constantly evolves through the different actors who play him. It is precisely for this reason that the character and feel of the whole show change constantly and keep a fresh feeling about it. Kudos to the writers for this as well for writing around the Doctor as created by the TV thespian.
Although the very fabric of what you expect changes over time, the similarities, that glimmer of familiarity between the chosen characteristics of a certain actor and his favoured Doctor(s) makes for a rich self referencing and familiar feel that will inevitably draw each of us to certain moments of the shows past that we’ve experienced. This unique aspect of the show allows different people to find the hidden references with which even the most observant and dare I say it obsessed of Who fans may have missed.
to define how a certain regeneration will work and stopping the show from getting stale also makes for interesting conjectures as to how certain other Doctors would have solved a problem were they in the place of the current one. which, amongst a myriad of other topics keeps all episodes alive with interest and debate long after the first decade of transmission. Then of course, allowing more than one Doctor to inhabit a story as has been done several times is to see a fun clashing of bickering personalities.
Each performance has to be – and has been – nuanced and varied, yet comforting as you see aspects of previous Doctors…it’s almost a continual strengthening of a single character whilst simultaneously being something new, different and separate from what has come before but with that thread of intimacy with the character.
The contrasting natures of each regeneration are often as fascinating as the similarities though, the stunts and Venusian karate of 3rd is at contrast with later Doctors who won’t touch weapons, for example. Of course this is also down to viewing tastes for the show as much as moralistic choices of the actors as well as the story arcs.
The main character thus drives the direction of show, which is built around him and to a lesser extent the companion or companions of the day, allowing the storylines to be somewhat symbiotic between revolving around the titular fella and the circumstances wherein which he be found. Regeneration, therefore is not only a clever idea but a way of keeping a show, theoretically going forever, same series, same production values, totally different feel and one epic time had by all.