The writer is generally not aware of the content of the writing until after the process is complete.
Traditionally, automatic writing (in its spiritual form), will involve the medium, the person the spirit works through, to be seated with a pencil, pen, quill and ink, or other writing implement in hand, and a large sheet of paper set out before them.
They may enter a trance-like state, allowing themselves to be more easily manipulated by a spirit (or other entity), who will control the mediums arm in order to write messages, symbols or even drawings, conveying the message.
Most times those who act as a spirit medium for this process are channelling messages from the deceased, but at times the messages can be coming from other sources, such as extraterrestrial life, or in some rare cases, other living people.
The writings can be in the form of answers to questions, guidance and even prophecies.
The Subconscious Mind
It is said some authors experience this as they fall into 'the zone', a state where everything works so perfectly that they are unaware they are even writing at all.
With much automatic writing being presented as symbols, or having the appearance of very rushed, very messy writing, skeptics point out it could just be the results of the ideomotor effect (another term for another time, but essentially involuntary/unconscious muscle twitches and contractions, ie movement), especially if a tool such as a planchette is being used.