Is it weird to say, "It depends on the story?"
Some stories come to me as plots. (There's a group of friends who go to a haunted house together for Halloween, but one of them wants to use the others to help him gain Vast Supernatural Powers. I've toyed with this plot idea for years, but the characters and their motivations and interactions keep shifting around.)
Some stories start with a character. (There's a young warrior coming of age, poorly socialized and nearly feral, who needs to learn more about himself and how to be more human. What needs to happen to get him there? Depends on what day you ask me.)
Some stories - possibly because I'm a fantasy writer more than anything else - start with a particularly interesting setting. (There is a massive floating city that sits on an inverted island half a mile above the ground: derelict, ruined, and waiting to be explored. Who goes there, and why, and what do they find as they explore? I'm not sure.)
So, plot or motivation? I'm honestly not sure. I think it depends on the story.
I did, however, get a very nice deal on chickens and eggs... I just can't decide which should come first!