10. Dont Let The Doctor/Companion Break Character
This may seem obvious but its vital to avoid this. Fans of the show will have expectations of how the companion and especially the Doctor will behave. The Doctor can be ruthless. He can allow characters to perish if they truly disgust him. He can passively arrange the downfall of his enemies. He can even take lives in self-defence as happened in Earthshock, and the Time War, presumably. But if he actively murders a character with brute force and little necessity, as he once did in The Two Doctors, that completely goes against his character. Similarly, if the Doctor or his companions act considerably differently from recent weeks, people will notice. I would personally say that the recent episode Nightmare in Silver suffered a little from this. Several people have commented that Clara received a sudden boost in confidence, enough to even command soldiers, which had not been alluded to or developed in prior instalments like Cold War. Similarly, The Long Game in 2005 concluded with an extremely harsh, almost vindictive act from both the Doctor and Rose. While the character of Adam was wrong to implant future knowledge in his brain, the manner in which the two main characters repeatedly clicked his head open and shut for a laugh and then left him with the threat of dissection smacked of cruelty. I could be wrong, but I would have thought that the Doctor would have at least ensured that no future knowledge could have been obtained through a simple click of the fingers.