When you say that someone is preconceived about something, you mean that they have a lot of ideas about what is going to happen, and they are not open to the possibility that something could be different.
- He was preconceived about the outcome of the game, and he was very disappointed when it didn't go his way.
- She was preconceived about the way the interview would go, and she was very surprised when it didn't go the way that she had predicted.