One may ask the same question in the title^ about actors or fine artists. Where in the artistic process and being does emotionality fit?
Seems to me that the subject is not how much the creative person feels, but how much empathy they have for others' feelings. Unless the writing is entirely about self, the power to put oneself in another’s overalls is key.
In addition to enhanced empathy, there’s something called mirror-touch synesthesia, where others’ physical sensations are literally felt in one’s own body. Some quantified version of this is operative in good writing.
There is a price to pay for being an empath. Just take a look at artists. The talented ones cultivate ways to express what they sense, thus producing books, painting, and theater performances. But this comes after the first principal: feel another's joy but also devastation.
Ah, the joys of being human.