There is no definitive answer to that question. Things come at you in different forms and ways - sometimes melodies, sometimes words, phrases or ideas. It's rare though that I sit down and start writing something without having had a flash of inspiration. I like to have a purpose, an objective in mind before I start writing / recording.... things can meander off into nowhere and become dull and boring if you're not careful. Then music becomes like work and it should never feel like that.