Doctor Who is usually pretty good at doing the creepy episodes. From the classic series with "The Curse of Fenric" or in the Big Finish Audios like "Chimes of Midnight".

Generally I find the creepy episode to be an enjoyable one. From Season 1's "The Unquiet Dead" all the way through to Season 9's "Sleep No More". "Hide" from Season 7 is probably closest in concept to "Knock Knock", with a spooky house.

The episode started quite strongly, with a suitably creepy looking house and David Suchet as the landlord, it was setting up nicely.
The death of Pavel was pretty good, not showing much and keeping the others from discovering his fate by keeping the music playing. His eventual death was another high point with his half absorbed form being pretty horrifying.
The death of Harry was also gruesome, with his contorted face visible as he was consumed.
These standout moments were just that, moments.

Sadly the episode failed to build any tension. There wasn't any spine tingling moments and it all felt a bit flat.
Part of this is unfortunately due to Bill who had her worst episode in terms of reacting.
I have been impressed so far that she doesn't seem to take things in her stride and seems genuinely interested.
This time she didn't convey any of the terror I would imagine the situation should generate. She seemed largely unimpressed most of the time.
Though in fairness none of the characters really sold the tension, if they aren't scared, why should I be?
The Doctor was ok, but didn't really add much to the episode and was a bit forgettable really.

So a disappointing episode, mainly due to the generally high standard these types of episodes reach.
5 times the real horror was Little Mix out of 10

Unfortunately, for me, KNOCK KNOCK was another case of a DOCTOR WHO serial that was a "filler" rather then a "pillar". I was more interested in what might be coming next week, rather than what had been on this week. Bill Potts continues to annoy the hell out of me. She's just recently shared three dangerous adventures with the Doctor - adventures that she could never even perceived as being possible in her wildest dreams - and yet she still lacks the basic common sense to realize when he might be on to something.

And as a fan of NCS: MANHUNT, POIROT and David Suchet's work in general, I have long anticipated his appearance in DOCTOR WHO. I even imagined what it might be like to have him spend a few years in control of the TARDIS. Having said that...

What a criminal waste of his talents!