If you prespawn the NPCs before the cinematic, you might want to turn the invisible. Otherwise the player will see them with force sense. Although not directly harmful, but it'd be nevertheless a technical glitch. But invisibility solves that quite easily. And like I said, the prespawning is OK to do just a little before the cinematic, so the NPCs won't really eat FPS for any significant period of time.
For example, if the player needs to walk to a room, and when he steps into the room through the door, a cinematic will launch. You can place a trigger little before the door. The trigger will target the NPCs that will spawn in a small, unreachable box belowe floors. They have a spawnscript setting them invisible (and any other things you might need). The player won't notice anything, but when he reaches the room, and the cinematic starts, you can just set_origin those NPCs to the room and turn them visible.
Smooth and clean. Or you can just run the cinematic from the NPCs spawnscript, or possibly target the NPC entity to a script_runner, which will run the cinematic (haven't tested this last one but should potentially work).
Frozen by ICARUS