This is a very interesting question as one of the major issues that anybody studying Shakespeare has to contend with is the fact that Shakespeare's plays were never meant to be read, but were meant to be seen in performance. At the end of the day, the presentation of plays and the characters within them is a careful choice of the director of the production, and, as anybody who has seen two different productions of a Shakespeare play can testify, you can often get two radically different presentations of a play or a character based on the same script. So, my first answer is that an actor cannot ultimately know how to play a character based on the script alone. The director of the production must give his impression of Lady Macbeth to the actor.
Secondly, however, it is important to realise that the script itself does yield a number of important clues that would help an actor in trying to play a character. The words and the stage directions are clearly guides for us to use, and inferences can be made based on these clues as to how to act out a particular character. For example, in Act I scene 5, where Lady Macbeth famously invokes the spirits to "unsex her" and make her relentlessly savage in the pursuit of her husband's gaining the crown, it is quite clear from the language that she uses that this is not a comic scene, and must be presented in a very grave, serious and earnest fashion. Thus, although the ultimate choice of how a character is presented is the director's, at the same time, the text can offer us clues based on the language used and the mood and atmosphere such language creates.