Discourse, expatiate, dilate, descant are comparable when meaning to talk or sometimes write more or less formally and at length upon a subject.
Discourse frequently implies the manner or attitude of the lecturer, the monologist, or the preacher; it may suggest detailed or logical and sometimes profound, witty, or brilliant discussion.
Expatiate implies ranging without restraint or wandering at will over a subject; it connotes more copiousness than discourse and often carries a hint of long-windedness.
Dilate implies a discoursing that enlarges the possibilities of a subject (as by dwelling on each small detail).
Descant stresses free comment, but it often also connotes delight or pleasure in this free expression of one’s opinions or observations.