【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.