Anne demonstrates one of human beings' greatest frailities - vanity. She is perfectly aware of Richard's evil and murderous nature, but Richard is cunning. He approaches her with calm sincerity, not attempting to deny her accusations, knowing that anyone who is denied (put on defense) only becomes more angry.
Richard then is the sweet-talker, first flattering her dead husband, whom she obviously loved. Richard states he was "fitter for [heaven] than earth.]" He then moves on to Anne herself, insisting to Anne that she is the one at fault. Her beauty drove him to it. Who could resist such attention? Anne is resistant, even horrified at first, but the sincerity in Richard's speech i stoo much to overcome. He does not over-exaggerate, nor is he off-handed in his remarks.
Finally, when her defenses become lowered, Richard abandons flattery for another human weakness - compassion, and sympathy. He prostrates himself before her, telling her to kill him if she will. Seeing another human being subjugate himself, seeing another human in pain, would only fail to effect the most cold-hearted amongst us. Anne is easily won over.
Lady Anne's seduction is quite unbelievable, and is perhaps one of the flaws in the play. One moment she is cursing Richard and loathes him for killing her father and husband, and the next she seems to accept his proposal.
Richard does not have appearance and physical beauty on his side, but the weapon he does have, is evidently much more powerful. Richard uses language and words to victimise himself in his exchange with Lady Anne. He twists each of her accusations against him quite subtly, so that they mean something else, which he then uses to further his own case.
Lady Anne is perhaps not seduced by Richard himself, but by his evil allure. In the play as a whole, Shakespeare has hit the nail on the head in terms of human psychology. Humans are attracted to the forbidden, attracted to evil. That is what makes Richard such a compelling and almost attractive character to the audience. It is likely that Lady Anne also succumbs to this same allure of evil. Indeed, she almost seems to be under a temporary spell, for when we next see her, she laments her marriage to the Duchess of York and Queen Margaret. That again brings me back to the point that her seduction is highly unbelievable because she changes her mind within moments! However, it is possible that although she hates Richard, her inferior position in society as a woman prevents her from doing anything but submit to him, and only when she is in the safe company of fellow women does she retaliate against him.