I'm one of those guys where everything flows from character.  Same plot and/or MacGuffin with different characters gives you completely different stories.  If it doesn't, your plot is pushing your characters, not the other way around. 

A runaway train is a plot.  Watching someone run away from a runaway train is a story.  (Or try to stop it, drive it, eat it, turn it into a poodle.)  So when I'm stuck writing a scene, or can't figure out how to move the plot forward, I tie someone's shoelaces together (figuratively).  Watching them figure it out while the train keeps coming usually provides what the story needs, or what they needed.

Not that plot's not important, clearly.  But people relate to other people, and that's characters.

That said, I know I'm ready to write when a) I know the final scene/reveal; and b) I've walked around with my characters long enough that I can hold conversations with them, out loud.  (My family's quite used to it, bless them.)  Literally acting out scenes in character while I walk around the house or whatever, and the characters say something that reveals something to me I hadn't considered that most always drives the plot forward in a way I hadn't imagined, that's better than what I'd planned.  That's when I know the coffee's ready.

Thanks, Oz!  Much appreciated!

Chris