Classic private eye novelist Raymond Chandler said that whenever he wasn't sure what do next, he had a man come busting through the door with a gun in his hand. Poet Richard Hugo, in his great book The Triggering Town, said that the time to change the subject was not when you ran out of ideas on the original subject, but before you ran out of ideas.
Don't ever be afraid to do this, as you write your novel. Don't be afraid to let in something you hadn't planned. Let someone come busting through the door with a gun in her hand. Let your character impulsively decide to have sex with her boss--or decide to go to the track instead of having sex with the woman he's fallen in love with. Have your character's best friend from high school quit her job, travel across country, and take up residence on your character's couch. Then change the best friend. You'd imagined her blonde, blue-eyed and stacked? What if she's Goth and a physics major?
Don't forget -- you can always cut it out if it doesn't work. But if you don't surprise yourself every now and then, you'll never surprise the reader.