The tragedy must be irreversible. The tragic personage is broken by forces which can neither be fully understood nor overcome by rational prudence. This is crucial. Where the causes of disaster are temporary, where the conflict can be resolved through technical or social means, we may have serious drama but not tragedy. When the bad bleed, the tragedy is good.
Comic scenes enlarge the canvas of tragedy.
Sometimes in fact, the comic scenes deepen the tragic effect. An aside is a sharing with the audience but supposedly not to be heard by the other characters on stage. These are two important conventions in Elizabethan drama revived in the 20th century. Adapted from Barnet, Berman and Burto A Dictionary of Literary Terms pp. Characters shown in dialogue, action and appearance.