That was the strangest thing - when someone told me that my writing was filmic. "I can see it all in my head."
Curious, because I can't see any of it.
I'm not sure what the reason is - perhaps it is an aspect of my dyslexia - but I don't see much in my head at all. When I am describing a scene in a novel, I am certainly in that scene. I know exactly where everything is. I could reach out and touch it. I could feel its texture, feel the heat and the cold. I can't see it though.
Here is the alchemy, then. The descriptions someone uses in writing a scene can act on the mind of the reader to produce a visual impression. With each descriptive detail, the mind of the reader is dipping into memories of similar things and assembling an image. Since people's memories are different, it seems reasonable to suppose that each person is "seeing" in their mind's eye a different picture.
Which is why adaptations of books sometimes upset readers. The image projected onto the theatre screen won't be the same as the image they projected in their heads when they first read it.
Maybe that's why they say the pictures are better on radio.