Acting involves portraying emotions on stage, which was believed for 1800 years to potentially make actors sick by becoming too attached to the emotions displayed. A Roman teacher in the 1st century AD created a system linking specific movements to emotions, still used today, recommending actors observe themselves with mirrors to understand. Later writers explored the exact emotions involved in acting and how they could be portrayed, with some believing motion creates emotion and acting requires an actor's spirit, not just rehearsed movements.