They're not embodied in that fictional world, and have no sensory perception of it. Their cognitive and decisional engagement with it is mediated almost entirely by words - perhaps accompanied by the odd illustration. Their emotional response to it is not triggered by actual sensations, perceptions, loves and hates, etc.
So whatever is happening, as far as RPGing goes, it is not going to be a process much like those parts of your life where you walk down the street, slip over in a puddle, feel relieved that you narrowly avoided being hit by a car, sleep with someone for the first time, etc.