Your audience, whether readers or viewers, likes surprises but not coincidence. Sometimes they can’t put their finger on the dissatisfaction but they feel it. How can you avoid writing a coincidence and putting in more surprises?
In real life your sister’s boyfriend may know someone in the D.A.’s office who tells him how the legal discovery process is going. In a story, this just doesn’t work. Your protagonist has to do the work. The supreme example of coincidence is the deus ex machina in older dramatic traditions where a god (deus) literally comes from a machine either through a trap door or lowered from the runways above the stage to resolve the story. The example most modern audiences might be familiar with is Don Giovanni’s father as the statue comes alive. Pretty contrived, huh?
Coincidences in your story feel the same way to audiences. Aw, come on, you expect me to believe that? So, keep your coincidences to a minimum. You are allowed one, and only one, per story.
Surprises, on the other hand, derive from the nature of your characters and your plot twists, even your setting.
A good way to create surprises is to work from your character background. You have done enough background on your characters to know how they behave in the norm. The surprises come from how characters respond when they are pushed. Ask questions like:
What is the worst thing that could happen to this character?
What is an action this character would not do?
Then give the character the worst and make her do something she would not normally do. Ask these questions for each scene. Add the surprises.
To add breadth to the story use surprises for characters other than your protagonist. The antagonist, the side-kick, the love interest, the best friend.
Creating surprises is a fairly easy exercise that will add tension and keep your audience involved.