You need a stealthy neural link hidden by hair, ideally on the motor cortex and cerebellum.
I take it a simple earpiece isn't going to cut it. Controlling movement is in theory very simple, you need to zap the parts of the brain that control movement with extreme precision. Motor areas of the brain are located in the frontal lobe. The motor cortex is the region of the cerebral cortex involved in the planning, control, and execution of voluntary movements. You'll also want to control the cerebellum as it helps with the coordination and movement related to motor skills, especially involving the hands and feet. It also helps maintain posture, balance, and equilibrium. They can be controlled via an implant or external neural link, either way these areas are hidden by hair.
You'll also need visual input and hearing, mainly the occipital lobe and auditory complex.
You can't control someone if you don't see or hear anything, otherwise they'll wobble awkwardly which will be a dead giveaway that the person is not being themselves. With precise brain scanning you could see and hear everything a person hears, without them feeling anything unlike the motor control which makes the person feel as if they are possessed. The areas of the brain that handle sight are in the back and hearing on the sides. In the same way these areas are also covered by hair, so there's no problem there.
For speech the broca's area.
Located in the front part of the left hemisphere of your brain. It has an important role in turning your ideas and thoughts into actual spoken words. Speech is complex so it will probably take a lot more fine tuning to get this working. Still can be hidden by hair (he's not bald is he??).
Facial expressions can be controlled by the motor cortex but also by the limbic system which may be harder as it's near the middle of the brain.
The limbic system is the part of the brain involved in our behavioural and emotional responses, especially when it comes to behaviours we need for survival: feeding, reproduction and caring for our young, and fight or flight responses. With this you could induce joy, anger or sadness just by pressing a button (for very realistic reactions). It also houses the reward system which is useful in conditioning the idiot to follow commands like a good boy by inducing euphoria and ecstasy. You can also use it to punish the idiot whenever he messes up. Be careful not to harm this part of the brain as it can lead to harmful addictions (limbic system fragile).
Overall you could control a person with relatively no ill effects, except of course the horrible sensation of being moved by someone else. Also note that if you don't inhibit the controlled persons motor cortex they will still be able to move while the pilot isn't doing anything. This can lead to some comical slapstick if you so choose.