This shared line provides the actor playing the Ghost with a brace of remarkably precise acting tips. More than tips, in fact; if a production retains these two lines, they confine the actor to a specific set of choices. It’s a great example of the dramaturgical hints that John C. Meagher describes in his excellent book, Shakespeare’s Shakespeare. Meagher’s premise is that Shakespeare the Playwright also served the King’s Men as Director, not in rehearsal but in the way he slipped stage directions into the plays’ dialogue.
So, the first line tells Hamlet Sr. how to respond to Horatio’s rudeness, while the second reminds the actor that it’s time to leave the stage, and how to make an exit. “Offended” could be interpreted a range of ways, and depending on the actor’s favoured style (or the style of the era), it could be a facial expression or a full-body response. “Stalks” is almost too specific, especially if one is wearing heavy plate armour. It connotes a plodding, determined, forward-leaning, slightly irregular march.
In past and modern productions alike, the lines impose some limitations on the special effects used to bring the Ghost on and off. The Globe possessed an elaborate winch system that could hoist an actor up to the “Heavens,” and there was likewise a trap-door to “Hell” beneath the stage, but if the Ghost is stalking, then the mustn’t have been floating or sinking — at least, not during this particular exit. Since Pepper’s Ghost at the latest, Elder Hamlets have been dematerializing via various stage tricks, and while I guess it’s possible to dematerialize while stalking, it’s just such a corporeal word that making the moment ethereal seems to contradict its spirit (pun intended).
I like the ambiguity of the Ghost’s offense, and for my own purposes, I particularly like the fact that Horatio’s final line to him invokes “Heaven.” But if the actor combines the two Shakespearean tips, then his overall impression in this moment should be less demonic and more dictatorial. Most likely, Hamlet Sr. is just annoyed that the guards brought Horatio, an outsider of no consequence, rather than the son with whom he’d rather chat. The subtext, as far as the Ghost is concerned, is “you’re not worth my time.”
As for Marcellus and Barnardo, the subtext is still, “We’re so terrified that we’re going to state the obvious.” And for the audience, the subtext is: “He’s leaving? Whaaaa?!?” as I’ll get into soon enough.
To Tweak or Not to Tweak: These are accessible, muscular, active lines, so you’d be a fool to change them unless you wanted a very different sort of exit for the Ghost, as I described above. In that case, you’re better off cutting them than changing “stalks” to “floats” or something equally on-the-nose.