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. The capacitance of a capacitor depends on the amount of charge. A positive charged capacitor plate only has positive charges on it.