Life is full of surprises--sometimes for better, sometimes for worse. Part of the enjoyment of reading a novel or watching a movie is being surprised by a turn in the plot or characterization. Of course it has to make sense, so that when we look back from that point in the story we can see that there's a reason for this change.
One place to look for a good surprise is the character arc. That's the transformation your protagonist experiences between the start of the story and the end. Not all stories have this but most do. It might be from selfish to giving, from closed-off to open, from afraid to brave.
As you may have experienced in your life, one change usually brings others with it. That's where good story surprises lurk.
For example, let's say your protagonist changes from closed-off to open. Some possible unexpected developments that might go along with this:
* his best friend, who was comfortable with the closed-off version, abandons or betrays him;
* his new personality doesn't match what he does for a living and he loses his job;
* he goes too far in the direction of being open and someone takes advantage of him.
You'll notice that these are, or at least seem to be, negatives. That's because when we're watching someone transform from a not-so-good state to a better one we expect to see the benefits. Just as in real life, we're not so prepared for the clouds that might come with the silver lining.
Of course it may be that in the long run it's to his advantage to have different friends and a different job, and being taken advantage of helps him to define his new limits. But when these first happen they probably shock him and us and give you some dramatic setbacks to work with.
(for help developing your writing from idea through to publication or production get a copy of Your Writing Coach, published by Nicholas Brealey and available from Amazon or your other favorite book seller.)
