Halo is a game about combat: it was very important to the development team that combat be compelling. They spent a lot of time refining the aiming controls, enemy behavior, weapon variety, and the layout of the environments to make combat as enjoyable as possible. The assumption was that if players approached the combat situations as designers intended, players would have a terrific gameplay experience. Researchers were called to test this hypothesis and see whether the typical player would have that intended experience with the game, and if not, what issues were blocking them. During testing sessions, players began shooting at the enemy as soon as they could see them, but the designers had wanted them to move closer, and then fight at a much closer quarter. At greater distances, the experience was less satisfying: e.g., the weapons were not designed to be effective at long range resulting in participants complaining their weapons were not accurate enough and were running out of ammo too frequently. In order to fix this issue, the designers implemented a contextual aiming cross at the center of the screen: the indicator would turn red when the player was targeting an enemy in the correct mode, that is, only at the combat distance intended by the designers. Designers also modified the behavior of the enemies: when shot from a distance, they would now find a safe spot to hide, requiring the player to move closer. Subsequent testing showed that these changes encouraged players to engage combat at the intended distance. The contextual cross is now a standard in the shooters genre.
Pagulayan, R. J., Steury, K. R., Fulton, B., & Romero, R. L. (2003). Designing for fun: User-testing case studies. In M. Blythe, K. Overbeeke, A. Monk, & P. Wright (Eds.), Funology: From Usability to Enjoyment (pp. 137–150). Dordrecht, The Netherlands: Kluwer Academic Publishers.