In the Outbreak titles, NPCs lack "clipping", meaning that the player and AIPCs can simply walk straight through them. They are normally completely still, and can only talk through subtitles via the 'Examine' feature. NPCs cannot use the "Trade" feature, so items "traded" by Ben Bertolucci will simply appear on the floor near him.
AI Player Characters are different from NPCs in Outbreak. These individuals can attack; pick up items; trade and speak, and are also solid. They are predominantly the other protagonists on the player's team when in Offline gameplay. The AIPC can also be an extra individual altogether if the scenario requires them. For example, Bob can be carried to the rooftop of J's Bar, and is vulnerable to zombie attacks.
In Resident Evil Outbreak File 2, the player has the freedom to select their own partners. To add a further need to strategise beyond character strength and speed, File 2 adds in personality. This is made more complex when one character likes another who feels ambivalent. Characters in a good relationship with the player will remain close by on the map; attack the same enemies and respond quickly to help and trade requests. Characters in bad relationships are prone to ignoring requests or even leaving the room upon the player's entering. Characters in ambivalent relationships will fluctuate between helping and ignoring the player.