There are only a few rules differences between how PCs and NPCs behave.
NPCs never roll - always have the PCs roll defensive checks rather than having NPCs roll their own dice. Only if the PCs wouldn't be directly involved, and you want to randomize the outcome, should NPCs roll dice.
NPCs cannot ever critically succeed, as they essentially don't ever roll dice. If a PC makes a defensive roll, and the opposition die rolls a natural 10, no critical effect is applied, it is simply a 10 and can be used to determine success or failure. As such, actions in the bestiary never include critical effects, and any basic action that an NPC takes that could have a critical effect, should simply ignore it. As the narrator, you may feel free to break this rule as appropriate, but the default expectation should be that an NPC is incapable of critical success - this is reserved for the PCs and they are the heroes (or at least, protagonists) of the story.
NPCs in general should not gain weakened conditions and test for unconsciousness once their breakpoint is reached; instead, most NPCs should immediately fall unconscious. This helps to expedite danger, making the game drag a bit less. Naturally, this gives an advantage to PCs, who are somewhat more resilient as a result. NPCs who should use the typical weakened condition rules are important NPCs (big bads) as well as any "lord" variant NPCs (e.g., the Bandit Champion surrounded by several Bandits).