English-speaking countries are governed by common law. In this system, the judge is always alone at the bench in the first instance, even if it is a criminal affair. His role is to manage proceedings, ensure that everything goes according to the rules.
He asks questions when the testimonies are unclear or if he wants a lawyer to justify his line of questioning. He can also call the attendance to order.
American judges always wear a black gown, while Australian and English judges wear different colours in different jurisdictions.