It’s an ever-fluctuating continuum. Sometimes an idea will pop into my head completely orchestrated and ready to be written down, and other times it’ll just be a little scrap — something I have to sit down and futz around with for hours before achieving any sense of clarity about what exactly I should do with it. Orchestration has always been my favorite part of the writing process, and I enjoy trying to figure it all out, digging and finding the information already hidden in a piece of musical material that will guide my decisions about how to treat it.
You can also vary the motif by lengthening or shortening the notes within it, and make it change direction by inverting it, creating its mirror image. You can add notes to extend it. And later in the song, you can even create a new motif, usually for the chorus. Contrast is the songwriter’s friend.