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After observing playtests, developers noticed players had trouble engaging the Screamer game mechanic effectively, and cut it from the game while incorporating some aspects into the Boomer's abilities - the Screamer's 'scream' was converted into the Boomer's horde summoning vomit attack.
From the developer commentary, it can be derived that players were able to engage both the Screamer and the Boomer with similar knowledge-based behaviors during their first encounter with each special Infected; with no prior experience of either type playtesters would treat them like normal special Infected until they were able to develop processes to deal with each one's special ability.
However, it was easier for players to understand how the Boomer worked and develop rules and skills based behaviors to counter it; players would learn fairly quickly that it was best engaged at range and that being close to a Boomer when it was killed was not a good strategy. This level of understanding allowed players to handle the Boomer using rule or skills based behaviors.
The Screamer, on the other hand, blended in with the regular common Infected before running away from the player to begin its horde-drawing scream; players may have been able to engage this game mechanic using knowledge based behavior but were unable to understand or develop effective processes at the rule or skills based level in dealing with the Screamer, which therefore lead to player frustration.
With the revised Boomer, players were able to go from knowledge-based behaviors of first treating it as a common Infected while going through the learning process of recognizing its shape, how it sounds, and the unfortunate effects of being caught in either its vomit attack or shooting it point blank.
From these experiences, players were better equipped to develop their own rule-based behaviors on dealing with the Boomer in order to defend themselves and their teammates by engaging at range, avoiding its vomit attack, and using the default "push away" melee attack to stay out of its post-death blast radius.
More experience would lead some players to develop the automatic, triggered automatic processes included in Skill-based behaviors to efficiently engage the Boomer game mechanics.
In developing Left 4 Dead 2, a sequel starring four new survivors working their way through a heavily Infected Louisiana, several changes were made to basic mechanics from the first game along with changes to environmental design.
Melee attacks now came with a cool down timer, forcing players who made liberal use of it in L4D to think tactically or endure a progressively longer wait between each swing. New weapons were also introduced, most notably hand-to-hand combat weapons such as fire axes, frying pans and katanas, while the players ability to use their unlimited-ammo pistols was removed in exchange for the melee weapon's close-range instant-kill attacks versus most common and special Infected.
Campaign-specific Uncommon Infected were introduced, namely to add some variety to the hordes of common enemy types and some minor additions to that aspect of game play as well; Hazmat suit-wearing Infected enemies encountered in the first campaign are immune to fire and Clown Infected during the "Dark Carnival" campaign draw small groups of Infected to them with their squeaking, oversized shoes.
Many game mechanics such as crescendo events and the special Infected enemy types carried over with few significant changes; experienced L4D players would already have a grasp of the established mechanics and thus be ready to face the repeat performances of the Boomer or the Hunter in the new campaigns.
The largest addition to the Infected rogues gallery were three more types of special Infected, introduced to counter player behaviors or actions that were commonly observed in experienced players following the release of the first game. Designers noticed that players, when faced with incoming hordes or dealing with a crescendo event, often would use enclosed spaces or corners to bottleneck attacking enemies.
The L4D version of the melee attack could be used to quickly defeat any special Infected that managed to break through or rescue teammates fairly easily. As team member Phil Co pointed out, "Left 4 Dead 1 had done such a good job of training people to stick together it was incredibly difficult to get them to separate." To encourage more creative game play, and to provide a new challenge to players, these new mechanics were introduced to help add more variety to L4D2's game dynamics.
Although the Smoker and the Hunter were already aimed at breaking up tightly packed teams, their effectiveness decreased when inside buildings or at environmental bottlenecks such as a hallway or door. Valve introduced the bulky Charger, a sort of miniature version of the Tank, which wields one massive arm to pick up and pummel a player into the ground to provide a counter to players' tactical use of such areas.
This special Infected, when attacking the player, first hurtles through the environment in a straight line at high speed, knocking anyone in its path aside until either impacting a solid object such as a wall or in some cases going straight off of a cliff or other level hazard. Often the first player to be caught in its way is dragged along for the ride, and when grabbed by the Charger's oversized fist, could only be rescued by killing it, as the Charger has immunity to a player's default melee attack.
Experienced L4D players would need to develop new rules and skill based behaviors regarding their environment and the safety of funneling enemies through doors, as the Charger introduced new risks to using those types of environmental bottlenecks. Chris Carollo noted that:
"In Left 4 Dead 1, long narrow hallways usually meant relative safety for the survivors. One player could cover the front and another could cover the rear, while players in the middle healed. With the addition of the Charger, however, the long narrow hallways in Left 4 Dead 2 turn out to be extremely dangerous."