When I'm learning a song I'll often speak it until the words come naturally and without thinking about them before I start singing. And that helps work out the narrative rhythms that are fitting.
And I do find myself picking up the guitar to try to help me shape the tune and get it into my head, if it's one I'm making up. But that's just laziness on my part, or maybe impatience. I know I shouldn't do that. It risks putting the tune into a corset, before it's grown into the shape it's supposed to be.