The wording I used is directly from the Dominate spell.
No, no, infiltrate them, watch them, and then sneak back to camp. Next time you meet them, use The Voice. You can use it at any time. You just have to have met them and heard them speak. Personally I'd love to have a DM allow it.Combat generally doesn't last longer than 3 minutes though, and if you can stay hidden that long you can probably get other ways to kill them.