Voltage is energy. Equipotential means equal field or uniform field. High voltage by itself is dangerous. It takes work to move a real charge on an equipotential. Charges move by themselves.
Sparks occur when an electric field pulls charges apart.
A potential difference is only on plates of a capacitor and not in region between. Charge flows through a dielectric, such as glass. No work is required to charge a capacitor. A capacitor requires two separate pieces. There is a net charge on a capacitor.