Peeps in your park interact with eachother. For instance, couples (generally teenage couples) will kiss. Families will argue. Peeps can also interact and react to different items that are put on or near rides. In Rollercoaster Tycoon 3, the interaction levels have risen drastically.
In the game you can group your peeps. Families can be created. They group in a circle and there is a group leader. The groups stay together.
In the game, there is a program that allows you to create your own group of peeps. These can be your family or your friends families. Each peep has their own traits and preferences. You can set a group leader. You can set looks clothing, and preferred ride intensity. This allows for limitless possibilites.
The game allows for cheat codes to be added. These codes can be put in for the name of a peep. Only regular peeps can have cheats. Staff members and VIPs cannot have cheat codes used.