Idiom: Backseat driver
Example: We are going on a road trip to the Grand Canyon next month. I am very excited about everything except for 1 thing: Joseph is coming with us. Joseph is a good friend, but he is a backseat driver. He tries to control the car even when he isn't driving! "Slow down, speed up, turn here, why are you driving like that?" I hope he will take a nap when I am behind the wheel.
Meaning: A backseat driver is a person who tells the driver what to do. This expression has slightly negative connotations as it implies that the backseat driver does not have confidence in the actual driver.