The world has been built, the characters have been created, and the route ahead is plotted. Now, to the writing. Before you get too far in, you’ll want to decide on your narrative style: what tense will you be writing in? Third Person? First Person? Is your narrator reliable?
If you’re not sure which to choose, you may want to try them out–maybe write the same scene using different styles and see how they feel. You might think at first that you want to write in first person but then realize that as you write, it becomes awkward or difficult to maintain consistently.
If you don’t make a conscious choice about what your narrative style will be, you may end up with a very inconsistent or uneven narrative. If you wait, you may create more work for yourself later down the road when you go back to edit. Those are the more practical/logistical reasons for choosing your narrative style.
The other reason this step is vital is it will help make the writing unique. This is what will set your story apart from other writers. This is where you find what will make readers fall in love with your writing and make them want to come back book after book.