Fawn, toady, truckle, cringe, cower are comparable when they mean to act or behave with abjectness in the presence of a superior.
Fawn implies a courting of favor by such acts of a sycophant as servile flattery and exaggerated deference.
Toady carries a strong implication of a menial as well as of a fawning attitude in an attempt to ingratiate oneself; often also it suggests the close following of a hanger-on or parasite or the vulgarly imitative behavior of a social climber.
Truckle implies subordination of self or submission of one’s desires, judgments, or opinions to those of a superior.
Cringe implies obsequious bowing or crouching as if in awe or fear; it usually connotes abject abasement.
Cower always implies abject fear, often cowardly fear, especially in the presence of those who tyrannize or domineer.