Some of our exercises use visual prompts. We track down images that just beg to have a story told around them. You pick a card that compels you, or we choose cards and then pass ours to the left. Again, surprise unlocks the words.
People in our groups get a prompt like "I know you, Joe (or whatever name you want to give your character), you're the person who..." and then you fill in the blank: hates Monday, leaves the dishes in the sink, licks a pencil's tip before he writes with it. When you get down to specific traits they spark an idea about a character. You read your selections and choose one or two to develop further.
We pass out index cards. Everybody writes one line of dialogue from a movie or a book, or even a conversation they overheard or remember. We turn our cards over and pass them to the left. Everyone writes another line on the blank card they just got. We pass the cards once more to the left. You now write for 15 minutes, using one or both of the lines of dialogue on the card you just received. It works because of the surprise.
If your book is stalled, you do some time-travel with this trick. Just cut to the good stuff to begin. That backstory might not be important to readers. Just to you.