I've written scores of songs just since I came aboard the Mudcat, some of them better than others, to be sure!! My experience is that you need to sorta capture the song -- the core image or even concept -- or the core of the vignette -- and sit there with it 'on your lap', so to speak, nibbling at your ear, as it were, until it transmogrifies into the line and the very word you need.
'Course sometimes you just have to nibble back. But sooner or later you find the delirious center which puts the best of the thoughts and feelings into the best words, and Bob's your midwife!! :>)