If you were to listen to a recording of "Surprise", you would hear that the only thing that really changes throughout is the level of volume at which the music is played.
At some points, the orchestra is playing as soft as possible, then will quickly gain volume and will play very loud, then will go soft again. This is how he created the feeling of surprise in the audience, since pieces that start out soft tend to stay that way and might only get a little louder gradually. "Surprise", however, will bounce back and forth between dynamics quickly.
