You are walking down the busy sidewalk with hundreds of people passing you. Some have headphones on and are in their own zone. Some are talking to each other within the crowds. Most, of course, are on their cellular phones, barely looking where they are going. As the masses shuffle past you, you probably will not speak to a single person. These are your non-playable characters. They are not for you, nor are they more than just background noise to your dynamics. They will not contribute to your journey, nor will they modify your path.
Most or some of your day is already planned out. You know where you want to go, what you want to do, and with whom you want to talk to and meet. There is a good chance you will not be making any detours on this journey, unless that is your direct intent. This means that you probably will not strike up a conversation with a complete stranger. As more people are engulfed by technology, most will probably not do this. Because of this, the course of your day is not affected by the NPC.
What if you went out of your way to disrupt your normal pattern and speak to someone entirely new? How would this affect the course of your planned and unplanned day? Would this instantiate a brand-new flow of events? Examples would include approaching up to a new person in the crowd, talking to a stranger on the train, or going out of your way to say hi to someone at work that you would not normally interact with.
What defines a non-playable character? They must not be directly controlled by you, there must be a large amount of them, and they must not alter events directly unless expressly acted upon by the main character. There are 3 categories in video games that follow the same idea. The main and playable characters, the supporting storyline characters, and NPCs. Interacting with the first category puts you in control of the scenario and the entire plot. You are able to be this character. Interacting with the second one alters events and storylines, supports the main characters, and are semi non-playable. The last category is essentially background noise. It is possible to interact with them, but they do not alter the main storyline and exist in large numbers as they are constantly regenerating.
In video games, everything is scripted. All categories, except the playable character follow some kind of logic and order. In real life, it more of a chaos problem rather than direct scripting. The 3 categories of characters from video games also directly translates into reality as you, your family and friends, and all other people, respectively. The main take away questions inspired by these parallels are as follows: In real life, how much influence do NPCs have on your story, would interacting with NPCs affect you directly or is does it serve as merely blank interactions, and how much effort is required to bring an NPC into your second-degree supporting characters?