Plot Deepening, like many other aspects of Emotioneering, can't be injected into a game near the end. It has to be woven into the story right from the beginning.
Here and there, you come across games with genuine depth to their stories, such as Ico or Final Fantasy X.
I'm not focusing on how "good" these games are; 20 people will probably have 20 different opinions. All I'm saying is that their plots had depth. There was even some Plot Deepening in the generally comic No One Lives Forever 2.
Whenever you feel depth in a story, be it a story in a game, film, or novel, that's because one or more Plot Deepening Techniques are used.
There exists a wide array of Plot Deepening Techniques. In fact, Ico, Final Fantasy X, and NOLF2 all use Plot Deepening Techniques not discussed in this chapter, as do many films, such as the Lord of the Rings trilogy and The Matrix. Perhaps you can do a little detective work and ferret out what techniques contributed to the depth of those plots.
Plot Deepening Techniques are an excellent way to enrich a game's story.