Following the Ghost’s first exit, Barnardo turns the scene’s focus back to the subject of fantasy, a word first used in line 22 (you remember line 22? Back in February?). Even though it was Marcellus who first used the word, not Horatio, Barnardo here seems to be throwing it in Horatio’s face, as if daring him to remain skeptical. This can be a useful acting note for Barnardo to carry back to the earlier parts of the scene. Not only is he tense and on edge because of his prior encounters with the Ghost; he is also resentful of doubters like Horatio who dismiss his experiences out of hand.
There is also an acting note for Horatio here, although it may not operate as an internal stage direction to the same degree as the ones we’ve already observed. When Barnardo observes that Horatio “tremble[s]” and “look[s] pale,” we certainly don’t expect to see Horatio yawning or inspecting his nails; but I would caution a modern actor against any attempts to literally tremble, since they will end up comical. And an actor cannot choose to “look pale,” although a clever director could send a request to her lighting director, to “pale up” the light on Horatio and the others.
What should an actor do, then, to make Barnardo’s observation work? For me, the means to creating believable emotional states onstage always start with breath. A bit of hyperventilation from Horatio — don’t start too early, or you’ll end up dizzy — will invest Horatio’s movements with an extra, slightly frantic energy. With advanced breath work, you can direct that energy into your extremities, to create a natural (not comical) quiver in the fingertips with each exhalation. For some actors, hyperventilation might create a flushed face — the opposite of “pale” — but if you rub a hand across your cheek just when Barnardo starts his line, you could shove that blush away for an instant — after which the blush will return, but will look like embarrassment, not physical strain.
If that approach to performance sounds too biometric, there’s a plain old metrical solution, too. The line just before Barnardo’s is “short,” ie. fewer than 10 syllables, as is the last of Barnardo’s three lines here. According to John Barton’s rules of Shakespearean verse (in Playing Shakespeare), short lines can provide actors with a chance for a small pause, to “fill in” the iambic pentameter with a few beats of silence. Here, we could imagine a pause (to catch the breath) just after the Ghost has exited. In that pause, Barnardo might reach out to touch Horatio’s arm — and Horatio might start and jump back. Then, after Barnardo’s question (“What think you on’t?”), Horatio takes another short pause to collect himself. Calm down, you can imagine him saying to himself, these men are counting on you.