When you start combat, all NPCs and characters that are in the combat move to 'turn based mode', but NPCs that aren't in the battle continue to have their turns in 'real time'.
I really hated this feature in Divinity games, and I hate it here. It breaks emersion, makes no sense, and is annoying. Here's a short list of where it matters:
1) In goblin camp, the floating eye thing doesn't aggro. It continues moving around near the frozen combatants, living as many turns as it takes the player to take his turn.
2) Whenever some enemies have not detected you yet, they continue having turns asynchronously to the turn based combat and move around until they happen to detect one of you and enter combat.
3) your invisible imp, any other invisible or hiding party member can spend as many turns as they want getting to position before they enter combat. Also their buffs/debuffs tick out.
4) you can send a party member out of combat range/escape, let her heal to full, go to camp, buy stuff, teleport to different waypoints, then come back to combat from whatever direction they want after spending as many turns as she wants dipping weapons on surfaces, buffing etc.
I understand it may have some merits for multiplayer (one player initiating combat doesn't interfere with another player kilometers away), but I would really appreciate being able to turn this feature off in my single player playthroughs.