I think that the exchange between Lady Macbeth and Macbeth in the last scene of the first act can help to bring out how easily influenced Macbeth is. He does show himself to be weak throughout this scene. At first, there is a sense of ambivalence that is all around Macbeth in this scene, as he shows himself to be someone that is not resigned to settling into action. When he initially objects to murdering Duncan because of being recently "honored," he is almost bullied into submission by Lady Macbeth. Macbeth, for his part, really offers no sort of opposing viewpoint to her when she passionately outlines the case for murdering Duncan. At most, Macbeth speaks of what were to happen if they "were to fail." I think that this shows weakness as it shows Macbeth to be someone who really is not committed to his previous stance of recognizing the honor that Duncan has bestowed upon him and the general sense of moral right that must be present in the way in which a chain of command is seen. Rather, Macbeth is more concerned with the consequences of failure. This is a location where Lady Macbeth shows more strength in her being able to assert that the courage to act might be as important as action in its own right. I think that this is where we can see some of the earliest evidence of Macbeth being quite weak, overwhelmed and incapable of resisting forces both within himself and outside himself that wish to do compel him to action that might be antithetical to what is inside him.
I wonder though if Macbeth remains weak...or if indeed he was as weak as perhaps some readings like to presume. Towards the end of the play, when Lady Macbeth is going mad, the audience sees a stronger and more determined Macbeth. Granted he does feel invincible because of what the witches have told him. But he shows little regard for his wife during the last scenes of her life.
Another point to consider is that whilst he was perhaps encouraged by his wife to commit the crimes initially, I am not so sure that she is so powerful as to completely influence his judgement. Admittedly until a recent viewing of the current RSC production of the play I was totally akin to the idea that Lady Macbeth was the stronger of the two. Yet actually Macbeth always had the strength in him, otherwise he wouldn't have dared to commit the crimes that his wife suggested. Did she help him? Yes. Did she influence him? Yes. But who is responsible? Him.
What constitutes his weakness is his failure to listen to his conscience/divine doctrine. He is driven by a lust for power, glory and status and will allow no one to prevent him from achieving his goals. His state of mind is thus ambitious and determined. These traits are not those of the weak minded, rather they are traits of those who have aspirations. His weakness lay in his inability to listen to reason and to let considerably evil notions to influence his judgement. Remember though that everything that is considered to be for the sacred honour of King Duncan at the opening of the play - when Macbeth is praised for the slaughter of Macdonwald! Yet ironically at the end he is cursed for his brutal behaviour when it is against the divine right of Kings. Perhaps then his weakness is that of a weakness of the spirit ... just like Eve in the Garden of Eden and the fall of mankind. Perhaps Shakespeare is pondering on what a man will do for power? A clear definition of weakness here is paramount ... because on Godly terms Macbeth was weak, totally corrupted by the forces of evil for his own self gratification. Yet by the acts of violence one could argue he is not weak at all physcially, but his insecurity and lack of self esteem gradually show his weakened state of mind as one by one the characters around him are killed off, to leave a King holding on to his power right until the end when it is taken from him.