I was talking with a blocked writer and I suggested the possibility of getting writerly flow by practicing writing--by writing a little everyday. This is basically the premise of Bolker's Writing your Dissertation in Fifteen Minutes a Day. Just write--something, anything, abut your dissertation--even just write the problems you're having writing. Anything just so that you write a little. The idea is that by practicing your familiarity with the writing process improves, and thus your ease of writing improves. One is to write anything at all--even "I have nothing to write"--just to get into the habit of putting ideas on paper. There is no right or wrong in such a scheme--or at least no right and wrong with respect to what you've written. You can write anything.
When I suggested this to the writer I was working with she said "But what if I don't write anything?" Writing is a choice. It is hard to write specific things, but if you can write anything at all with no prejudice as to what is written and no judgment about what is written, then writing is a choice and nothing more.
You, the writer, must sit down to the project. And you must choose to write something. But if anything written is a success, then there is nothing but the choice as to whether you will put pen to paper.
Yes, you have to choose what to write, but if you're writing for the sake of writing and practicing writing then anything that you do write is right. If you can't think of anything else to write, you can always write "I can't think of anything to write." That's where it start--by writing anything at all, by recording your thoughts--whatever they are--on the page.
If you practice that and the process of writing becomes more familiar, you'll think of other things to write as you go. But you have to start by choosing to write.