The point of this shake up is to keep viewer/reader interest through the middle of the story. At this halfway point, a reader might start getting too comfortable about what's happening in your story. Don't let them do that...keep them on their toes. Shake things up.
The midpoint usually involves a change in both character and plot. It is also sometimes called 'The Reversal'.
Take a movie you know well and look at what happens halfway through. If you have the movie on DVD, simply look at the running time and skip to the scene that is at the halfway point. You will probably see that something happens to shake things up or reverse the plot, or reveal something important that will have the characters change direction or try something new to solve their plot problem.
A good illustration of this is the 'Shark City' scene in Jaws. Up to this point in the movie, we have learned the folllowing pieces of information:
*there is a great white shark hunting along the beaches of Amity
*the mayor won't close the beaches because the town needs tourism dollars
*the mayor won't hire Quint, a local fisherman to catch the shark
*Chief Brody does what the mayor tells him and keeps the beaches open
A scene occurs to spark the midpoint...the shark attacks and kills a boater in 'the pond' and Brody's son Michael is in danger.
Then a total reversal happens. In the hospital, Chief Brody tells the mayor (who has a minor breakdown) that he WILL close the beaches and he WILL hire Quint to catch the shark.
There's only a small piece of this scene on Youtube but if you have the DVD, this is the scene you're looking for...
Now the movie changes and the action shifts from the town and the beaches to Quint's boat and the open sea.
The film Alien starring Sigourney Weaver and Tom Skerritt has a running time of 117 minutes. Half of 117 is 58. Here is what happens at the 56 minute mark in Alien...don't look if you're squeamish)...
Now the crew have to fight the alien onboard the ship.
Shake Up Your Series
So what does this mean to your series? Just as I said in an earlier post that the serial structure is embedded into our subconscious through the medium of TV, the midpoint is also part of the same inate understanding. If a midpoint shake up isn't there, you may risk losing your audience or having them think that something is missing from your story, even if they can't define what that something is.
If you're writing a 6 part series, your midpoint will happen in Episode 3. Take a look at that episode. What can you do to shake things up and keep the reader from geting too comfortable in your fictional world? What can change, either in the plot or in the characters (or both)? Is there anything that can be reversed? Can you throw a curve ball at the reader?
In Season Two of Angel, halfway through the episodes, Angel fires the rest of the group and goes off alone. Cordelia, Wesley and Gunn have to run Angel Investigations on their own without Angel's help. Meanwhile, he goes through a dark time and becomes involved with Darla, which triggers a series of flashbacks that explore his characgter more deeply.
Think about your own series and shake things up.