(above) Cranach Press edition of Hamlet (1930), William Shakespeare, Public domain, via Wikimedia Commons
(left) a dubiously legal pdf I found online of Rosencrantz and Guildenstern are Dead by Tom Stoppard. This absurdist comedy elevates these minor characters into the main focus of an existential drama.
Guildenstern is largely less able to keep up with the rhetorical turns made by Hamlet as compared to Rosencrantz. Most notably, the recorder exchange where Hamlet urges him to "play upon this pipe" (page 30) because it is "as easy as lying." See below for how I imagine Guildenstern might have attempted to please Hamlet via recorder.
Marcellus is a member of the royal guard who first sees the ghost of Hamlet Sr. alongside Bernardo. Why he chooses to call Horatio about the ghost problem is a bit of a mystery, though some scholars have speculated that this stems from Horatio and a Captain of the Guard character being combined in print. I feel that this is a boring answer, and rather that Marcellus calling Horatio instead of a royal guard captain implies something important about their relationship. I will leave the details of that open for actors to discover, and will just note it as an interesting feature while we're here.
Scene from Hamlet, Act I., Scene IV., Henry Fuseli (1786), Public Domain via Wikimedia Commons
Both Marcellus and Guildenstern are of lower social standing than the royal family, though their allegiences within the family are quite different. Both seem to care for Hamlet's well-being, though Marcellus does so out of duty and Guildenstern's seems to come from friendship, Guildenstern's is ultimately the most variable due to Claudius's nudging. Though Marcellus is not a scholar, he is sometimes more astute than Guildenstern; able to recognize the dangerous terrain the appearance of Hamlet Sr.'s ghost portends and act accordingly, while Guildenstern remains largely guileless, sent to his death in England.