Just like the linesman refereeing offside is forced to make his arbitration on the basis of peripheral vision and a sort of imaginative joining up of the dots, so the reader or viewer ascertains the most important truth in the subject matter from not being told something straightforwardly but from being invited to imagine it. But it's the act of imagining that invests it with importance: revelation through intuition is just more powerful.
Last week (can't remember where) I read that in Pride and Prejudice it is nowhere stated that Mr Darcy is dark. But every reader knows he is; in fact it's the most important thing about him really. It makes him an archetype.
Another example: in the Tony Richardson film of Look Back in Anger, in the course of one of his rants Jimmy, as played by Richard Burton, turns to the fireplace, away from the viewer, puts his hands on the mantel-piece and just rolls his shoulders. You don't hear anything and you don't see very much at all but it's enough for you to realise that Jimmy is still a scared little boy and it's this pity that makes you want to forgive him his behaviour. Without this forgiveness he's just a bully and the play doesn't work (as it often doesn't).
John Gray in Straw Dogs wrote quite a lot about this unconscious apprehension of things. We do it an awful lot apparently.
UPDATE: Writing this has made me realise why The Kindness of Women by JG Ballard is one of my favourite books. It's perhaps an unlikely word to use of Ballard but it's his emotional reticence that makes the book so lucidly beautiful and affecting.
I've also realised that filling in the gaps isn't an act of imagination: that would be too instrumental. It's more that what we conclude just occurs to us, as an intuition.