Suppose you were writing a book review. If you are anything like me, you would be thinking about what to write as you read the book, with actual words, phrases and even sentences forming in your head. Writing,then, is a mental activity. you have to write it down to know what you have, and also so as not to forget it, but the main work takes place in the brain.
I only point out something so self evident because I think the writing down is an obstacle for some. the student who can talk but not write, for example. I think that I could write just as well by dictating my words as by typing them or composing with pen,
When I write a poem, I think it up in my head and write it down later.
So perhaps the issue is one of memory. A working memory so weak that it can contain only a few words at a time?