The Subject of 'Uncanny'

The subject of the uncanny is when something appears strange or not right in the mind of a human being.

The unfamiliar and frightening or the familiar mixed with the unfamiliar

Intellectual uncertainty

When something is different, unrecognisable or strange in the way it looks, sounds or behaves etc for example a person coming back to life when you think they're dead is a common fear. Also when people are shown as coming back to life they're not normally themselves as they were known to others before the event of death so this adds to the weirdness still. They were familiar but become unfamiliar.

Unheimlich is opposite to heimlich meaning familiar 

It is something that is familiar but also strange at the same time like a talking doll – the doll is something quite recognisable and known to us but it having the ability to talk is strange as it goes against everything we know. Also things like doppelgänger  the German translation literally means ‘double goer’, which is a double of a living person that is thought to be paranormal and therefore sinister. In most films involving a doppelgänger the one that isn’t the actual person is often portrayed as the bad one or there will be something not quite right about them like the behaviour will be odd or the way they talk.

The uncanny has a lot to do with state of mind and when something strange does happen that appears to be uncanny it’s normally seen as something happening within a person’s mind or their own perceptions rather than something that is actually happening in reality. Sometimes it is deliberately made ambiguous for example if you were an audience watching a film you wouldn’t know whether it was all just a dream, a figment of someone’s imagination or whether it was actually real and this leads to a greater sense of unease for an audience member because people would normally rather know something than not know.

"Jentsch says: “In telling a story, one of the most successful devices for  easily creating uncanny effects is to leave the reader in uncertainty whether a particular figure in the story is a  human being or  an automaton;"

Sources:

http://www.bbc.co.uk/programmes/p00vdxt5 - Mark Gatiss