[In an in-depth Gamasutra analysis piece, Ellison looks at the universe and history of player-NPC dialogue interaction in games, analyzing titles from Mass Effect through Facade to The Sims and beyond.]

In 1966, Joseph Weizenbaum created ELIZA (pictured below), a computer program designed to emulate interaction between the user and an artificial therapist. Ever since, designers of interactive entertainment have attempted to incorporate meaningful interactions with virtual characters in order to aid immersion.

The most popular Western RPGs, like the Baldur's Gate and Fallout series, live and die by the strength of their dialogue and the player's ability to influence the non-player characters. Japanese romance games such as Konami's Tokimeki Memorial and visual novels like the Phoenix Wright series revolve almost entirely around character interactions.

Even action titles like Grand Theft Auto: San Andreas incorporate elements of non-violent character interaction to make the world come to life. However, while a great deal has been written about the process of creating game characters and writing for games, very little literature has addressed the mechanics behind character interaction in games.

While likeable design and well-written dialogue are among the most vital aspects in engaging a player with in-game characters, the systems of interaction certainly play an important role as well.

NPC interaction gameplay is a part of nearly every modern game, in the form of dialogue, barking orders at followers, or just choosing who to talk to in town. However, the western games industry has always found it difficult to create successful games based solely around character interactions, even though such things are popular in other media (television, novels, etc.).

The difficulties in interactive conversation lie in giving the player the illusion of freedom while still feeling natural and driving the story forward along interesting paths. Finding the most interesting and engaging way for a player to interact with game characters and develop relationships potentially opens up a wide array of game concepts and themes not typically explored by western game developers.

NPC interaction gameplay has seen numerous permutations throughout the years, but most can be separated into a few categories based on common design patterns. Gameplay varies with regard to level of interactivity, interface, and how the game presents the player with his potential responses. What follows is an overview of the gameplay of character interaction to date.

Non-Branching Dialogue

In games using non-branching dialogue, the simplest form of interaction, the player walks up to an NPC and initiates conversation. The NPC delivers his or her lines and the conversation ends. Alternately, initiating a conversation with an NPC triggers a cutscene where the player's avatar and the NPC have a non-interactive dialogue.

If the player talks to the same NPC again after certain events, the NPC may have different things to say, but the player never has any control over the conversation. The only decision-making involved is whether or not to talk to the NPC.

This type of interaction is extremely common and easy to implement, and can therefore be seen in almost any game featuring non-hostile NPCs, such as the original Final Fantasy and The Legend of Zelda: Ocarina of Time. While non-branching dialogue is still a regular facet of modern games, due to the absence of gameplay it is included in this article only for reference.

Branching Dialogue

Game dialogue becomes more interactive when conversations can take different paths. The player reads dialogue and chooses their response from a limited set of choices available to them. Conversation typically moves forward such that the player cannot go back to previous topics or responses.

From this basic framework, NPC interaction can be as simple as the player answering a yes or no question in a three-line conversation with a random NPC in town, or as complex as the relationship-building simulations in Japanese dating games like Tokimeki Memorial.

In games where branching dialogue is the primary gameplay focus, the player's choices often affect the NPCs' attitudes toward the player one way or another, with the player attempting to guess the "best" response in order to maximize NPC disposition.

One common technique employed to give the player a greater illusion of freedom is to have multiple responses lead to the same path. This is usually done as an attempt to limit the quantity of dialogue that must be produced for the game. Therefore, branching dialogue usually curves back in on itself such that while an individual choice may immediately produce a unique response, the rest of the conversation is typically not unique to that choice.

In games where the goal of conversation is to improve the player's relationship with the NPC, however, while every choice may not change the course of the dialogue, each choice may have a different number of "mood points" associated with it, and thus the player must still carefully consider his options at every turn.