If everyone is buying into something that is holding them back from solving the problem, the MCs doubt will allow him/her to break through that barrier. When a prosecutor (?) doubts that a witness has shared the whole truth, so help him God, the prosecutor digs a little deeper into the witness's story and uncovers damning information. When a smooth talking stranger makes big bucks by promising miracles with his miracle salve, the MCs doubt has them peeking behind the curtain to find that the stranger is just a snake oil salesman. When Jack promises a guaranteed way to make easy money fast, Jill's doubt has her looking into his past and finding a history of Ponzi schemes that bleed others dry.