The fact that only Macbeth perceives the ghost of Banquo in Act III, Scene 4 indicates to many critics that Banquo's ghost is the embodiment of Macbeth's guilt, a manifestation of his sub-conscious, much as the bloody dagger of Act I was before he killed Duncan. Thus, the ghost is employed by Shakespeare to indicate the paranoia of Macbeth, a paranoia which will lead him to commit other rash acts such as his stating before Ross and Lennox and the other lords,
Thou canst not say I did it: never shake
Thy gory locks at me. (3.4.63-64)
If Lady Macbeth were to see this ghost also, she may not have encouraged Macbeth to not think of the ghosts; she may not have been able to excuse Macbeth's behavior so glibly. However, whether these two factors would make a great difference upon the future events of the plots is dubious. For, Macbeth has already aroused the suspicions of the other lords of Scotland with his words that are quoted above even if his paranoia were reduced by Lady Macbeth's having seen the ghost as well. Besides, it is soon after this scene that Lady Macbeth who is not as forceful in the last two acts as she is in the first three, herself succumbs to the agonies of her own conscience in Act V. Therefore, mad as she becomes, Lady Macbeth can alter what Harold Bloom terms "devouring time" no better than can Macbeth himself, and neither is sure of anything but the present moment. And, in the final act, Lady Macbeth loses her certainly on even that as Macbeth loses his hold upon his reign. There is no character who has power of mind over the universe of death in Shakespeare' play.