GDC 2014 Blog Header

If you were to ask most gamers what their favorite aspect of Bioshock Infinite was, it would almost certainly be the characters and the story. Narrow that question down to their favorite character, and it’s virtually always going to be Elizabeth. She’s emotive, funny, and a great counterpoint to the stern, harsh, and somewhat unsympathetic player character, Booker DeWitt. But getting her there took a huge amount of effort, and it’s not just her dialogue and visual design that did it; the AI played a huge role as well.

John Abercrombie, Lead Programmer at the now sadly defunct Irrational Games, gave a talk at GDC 2014 about designing her AI to make her not only seem more lifelike, but also how to do so without her interrupting the gameplay. The first idea was to make Elizabeth a bigger part of the game by ensuring she was in front of, not behind, the player. Most AI controlled partner characters tend to trail behind the player, letting them take the lead. By putting Elizabeth up front, she’s in your line of sight and difficult, if not impossible, to forget. Guiding the player along, she seems like a more active participant in the game.


Placing Elizabeth was also a major concern, and the team implemented an idea called “Goal Side” to ensure that Elizabeth was always placed to the side, between the player and the checkpoint/goal. The term stemmed from Football/Soccer, where the defender is always expected to be between the ball and the edge of the goal they’re defending. Even then, distance was a major concern: too close and she would seem creepy, too far away and she would have a hard time anticipating the player’s movement.

One of the best touches to Elizabeth’s AI was the way she interacts with the environment in such a natural way, stopping to lean against walls, sit in chairs, examine things, or listen to a conversation between NPCs. This, of course, was not a coincidence. John explained how his team built a system called Smart Terrain in order to automatically place points of interest around every map the team was generating, such as walls, chairs, and the like. The team then went back and added additional points of interest, such as a painting of Lady Comstock or an important book, by hand to help strengthen the illusion and make it feel less formulaic. This extended to Elizabeth being able to identify items for Booker on the map as well.

Combat presented an interesting challenge, as Elizabeth initially took cover in areas that would be very unrealistic and frequently got in between the player and the enemies. An additional challenge presented itself when Elizabeth would throw the player items, as the player could continue to move, and a thrown item could be blocked by a piece of level geometry. An early version of the game required the player to walk up to Elizabeth and retrieve the item. Because the action of the gameplay generally demands most of the player’s attention, the team decided to be more liberal teleporting Elizabeth around in these cases, freely moving her once she was out of the player’s line of site.


John and his team also took a lot of tips from traditional stage drama, using blocking, lighting, and exaggerated movements to draw the player’s attention to what they need to see. Elizabeth initially pointed to enemies when they arrived, but they found that players were too busy fighting to bother to look at Elizabeth to see where she was pointing. This was later switched to the final system that makes far-off enemies, like snipers, easily identifiable via icons and a spotlight effect, a very dramatic change indeed.

One of the biggest takeaways was John’s insistence that “If the player didn’t see it, it didn’t happen.” It’s a truism, and the team’s choice to ensure that Elizabeth was visible virtually all the time and interacting with the world around her in a believable way was a major part of what made her so endearing.

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