In Canto 3, as Dante and Virgil stand on the "melancholy shore" of the River Acheron, the last barrier to their entrance into the underworld proper, Dante describes the approach of Charon:
And here, advancing toward us, in a boat,/An aged man--his hair was white with years--/was shouting: "Woe to you, corrupted souls! . . . . And you approaching there, you living soul,/keep well away from these--they are the dead." (3:82-90, Mandelbaum trans.)
Charon, the boatman, sometimes depicted as a minor demon but more often just as a wild-looking old man, quickly recognizes that among the dead waiting for transport to Hades is a living being, Dante. Charon's first reaction is to warn Dante that he is among the dead, who are drawn to the living because they sense the spark of life around them and, as we see later in the poem, are actually drawn to the warm blood of a living person (not like vampires but still to be avoided).
Because Dante makes no move to step aside, Charon tells him that
'Another way and other harbors--/not here--will bring you passage to your shore:/a lighter craft will have to carry you.' (3:91-93, Mandelbaum trans.)
Charon's reaction--believing that Dante is there by mistake--is natural: not only is the Underworld reserved for the dead but there is also very little precedent for a living being to journey to Hades and return to the upper world. In classical myth, we have only a few examples of the living making a journey to Hades and returning--Aeneas in Virgil's Aeniad and Odysseus in the Odyssey, for example. Although Virgil belongs in this realm, Dante does not.
After Virgil tells Charon that "our passage has been willed above" (l. 95)--that is, by Beatrice as divine spokesperson--Charon returns to his main task, which is to load the souls into his boat for their last passage, and he carries Virgil and Dante to their unnatural visit to the Underworld.