What are some external forces (characters) that affect Macbeth?
The witches affect Macbeth throughout the play. It is only after meeting them, and hearing their prediction that he will be King of Scotland that he begins to plot his murderous course to the crown. He consults with them again in the play, and Hecate makes it clear that it is she that is responsible for summoning the apparitions that gave Macbeth a false sense of security through their prophecies that he could only die when Birnam Wood advanced on Dunsinane and then only at the hand of someone not born of woman. Before predicting that Macbeth would consult with them on his destiny, he says that she will:
...raise such artificial sprites
As by the strength of their illusion
Shall draw him on to his confusion.
He shall spurn fate, scorn death, and bear
His hopes ’bove wisdom, grace, and fear.
It is this prophecy that gives Macbeth an overconfidence that plays a role in his demise.
Lady Macbeth also plays a pivotal role in encouraging Macbeth to commit the murder of Duncan, the act that sets him on the path to ruin. When Macbeth wavers before killing Duncan, she questions his manhood:
Art thou afeard
To be the same in thine own act and valor
As thou art in desire? Wouldst thou have that
Which thou esteem'st the ornament of life
And live a coward in thine own esteem,
Letting “I dare not” wait upon “I would”
Like the poor cat i’ the adage?
Ultimately, though, it is Macbeth's decision, and his alone, to kill Duncan and by the time he orders the murder of Macduff and his family, his wife plays very little role in his actions. His ambition, combined with fate, is always the main source of agency in the play. By Act IV, this deadly combination is the only driving force.
My quotes are better.
Lady Macbeth: "When you dirst do it then you were a man, and to be than what you were, you would be so much more the man."
"The castle of Macduff I will surprise; seize upon Feif, give th'edge o'th'sword his wife..."