Frank, candid, open, plain are comparable when they mean showing in speech, looks, and manners the willingness to tell what one feels or thinks.
Frank stresses lack of reserve or of reticence in the expression of one’s thoughts or feelings; it therefore usually connotes freedom from such restraints as fear, shyness, inarticulateness, secretiveness, or tact.
Candid is often used interchangeably with frank; it may distinctively imply a fundamental honesty and fairness that make evasion impossible and suggest a refusal to dodge an issue or to be governed by bias or fear.
Open implies both frankness and candor, but it often suggests more naturalness or artlessness than frank and less conscientiousness than candid.
Plain comes closer to candid than to frank, but it suggests outspokenness, downrightness, and freedom from affectation more than fairness of mind.