While you have a plot circling in your thoughts to make the plot work, you need characters. Now you could start with just a single character but what impact does that single person have on the plot. The single character needs to relate. In the relating to incidents in the plot, having a second character is a help. Especially when the two characters now have the start of a relationsip. Questions abound. How do they react to the first meeting and to each other.? How do they feel about each other? Do they come together to become one immediately or are there problems in building the relationship. But often when there are two characters nothing happens. They can like, hate, be indifferent to each other. Then add a third character and you've increased the number of relationship possible and added to the reactions to the event. So characters, the right ones with the right problems are essential to making the plot enfold them and make the action seem right.