Some Doctor Who producers / directors have been better than others at controlling the actors and insisting they play straight and take their part seriously. That is why there seems to be a greater preponderance of bad performances in the 80s under JNT than say, the seventies under Barry Letts and Phillip Hinchcliffe, who were both strict about the show being taken seriously. Once Hinchcliffe leaves you start to see more frivolous and badly judged performances creeping in; it didn't help that actors took their license to send the characters up from the humour infused into the scripts by Douglas Adams and the quirky moments supplied by Tom Baker, missing the point that:
Douglas Adams scripts were witty and amusing and they were not.
Tom Baker had given himself license to play the part that way by establishing the character so well - whereas the guest actors were better advised to play straight.
I will never do anything other than despise Bruce Purchase's performance as the Pirate Captain in 'The Pirate Planet'.
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In the early days of the show a lot of the actors would be theatre actors relatively or entirely new to TV. One good example of this, discussed in the DVD commentary for the serial, is the character of 'Yartek' in 'The Keys of Marinus', who had apparently been unreliably informed prior to shooting this first TV work to never blink whilst on camera, making his character's intense stare very disconcerting. The actor, Stephen Dartnell, would reappear in 'the Sensorites' as 'John', doing slightly better.
On top of that the show was more or less recorded 'as live', with only the occasional retake and stop & edit to insert special effects sequencies. In that format fluffs and slips were inevitable, especially in the inadequate studios the show was filmed in.
Anything other than a forgiving attitude towards some of the performances, in the circumstances, would be churlish. It would be interesting to see how the actors of today would fare in the same conditions. I honestly doubt they would be any better. In fact, I'd love to see the modern Doctors remember all of their bafflegab and never fluff it working under the same conditions as Hartnell did, and with arteriosclerosis no less, as someone already pointed out.