As with any film, the director must make a final decision as to which human being will portray a fictional character. Thus Polanski chose the thin, pale, almost waiflike actress Francesca Annis to portray Lady Macbeth. She has an air of innocence, yet sex appeal as well, particularly in scenes where she is partially clad. The biggest differences between the characterization of Lady Macbeth in the play and in the movie is at the beginning. As the character weakens in the play, the both portrayals come closer together.
The Lady Macbeth of Shakespeare's play is presented as a seemingly tough woman, mentally and psychologically at least, that can bully her husband (a battle hero) into doing her bidding, including regicide. She seems to be more critical and more manipulative in the film whereas Ms. Annis plays the character as more whiny and childish.
As the play progresses and Lady Macbeth descends in to madness, the differences become less pronounced.