In Shakespeare's Macbeth, Lady Macbeth's two soliloquies in scene five of Act I partly demonstrate why Lady Macbeth is a highly prized role among female actors. These speeches reveal a complex person, to say the least.
Lady Macbeth shows she is worried about her husband's scruples: he may not be willing to kill a king in order to inherit the thrown! His nature may be too "full o'th'milk of human kindness." She knows he wants the throne, but may not be willing to do what's necessary to get the job done. This, of course, shows not only her own ambition, but her ruthlessness (her personality). She is willing. She is anxious for her husband to "Hie thee hither," get here quickly, so she can impose her will on him and get him to do what she wants done: assassinate the king (her plans).
In the second soliloquy (36-52), she cannot believe her luck: Duncan is coming to the Macbeth castle to visit and spend the night! His entrance will be a "fatal" entrance. Even the raven is "hoarse" at the news. She begins to fortify her own willingness to go ahead with her plans:
...Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe top-full
Of direst cruelty! Make thick my blood;
Stop up th'access and passage to remorse,... (38-42)
Lady Macbeth pleads to have any female characteristics removed that might stop her from killing Duncan, pleads to be filled with the worst cruelty, and pleads to have her blood made thick, to block emotions such as pity and fear from operating within her (according to notes in my Norton critical edition on the play). She also pleads for the darkest smoke to cover her coming deeds so she cannot see the the wound her knife will make, and heaven cannot see and stop her.
Concerning Lady Macbeth's belief-system, she repeatedly refers to "spirits" in both soliloquies. These spirits are evoked to aid her in her quest. Whether she really believes in these spirits or is being metaphorical is ambiguous, and I'll leave that up to your interpretation.
In Act 5 Scene 1 of the play Macbeth by William Shakespeare, Lady Macbeth talks of her murderous plans and appears to call on evil spirits to assist her. (The raven, also, was thought to be a bad luck symbol - a bird whose appearance was a bad omen of dark times ahead.) She asks for the better or feminine, gentler part of her nature to be detached, presumably leaving her with only bitter, negative and evil primeval drives, using the words "unsex me." This shows her high intellectual capacity and that she (arguably, this is unlike Macbeth later) still has the power of reason and therefore the motivation and capability to commit sin. In saying "make thick my blood" she is showing the profound evil in her persoanlity as she seems to be requesting that sympathy, empathy and any "milk of human kindness" does not reach her heart.