Forthright, downright are comparable because they agree in their basic sense of moving or in the habit of moving straight to the mark.
Forthright (see also STRAIGHTFORWARD ) applies to whatever gets its effect by a straight thrust as if of a sword driven by the arm of one person into the breast of another; it therefore usually connotes dexterity, directness, straightforwardness, or a deadly effectiveness.
Downright, on the other hand, suggests a falling down or descending with the straightness and swiftness of one who leaps from a cliff or of a weapon that delivers a crushing blow. The word, therefore, usually implies crude force rather than dexterity, and concern for the effect produced rather than the point reached; often, in addition, when applied to persons or things it connotes plainness, bluntness, flat-footedness, or an out-and-out quality.