After the midpoint, a screenplay often loses its way. This is the dreaded second act sag. A couple of techniques to avoid this letdown:
1. Introduce a new character.
2. Introduce a new tension.
In older films, you will notice that rich, varied characters will often be introduced more than halfway through the film. For some reason, modern films shy away from this. But it's a perfect way to give your screenplay new direction and life.