 "Gehenna" marked the first appearance by Chris Ellis as Group member Jim Penseyres; Ellis would go on to reprise the role in "Dead Letters" and "The Judge".The episode opens with a quote from the Anglo-American poet W. H. Auden, taken from the 1939 poem "Blessed Event"—"I smell blood and an era of prominent madmen", the final line of the poem's third stanza.Poetry by William Butler Yeats, a contemporary of Auden's, formed a plot point in "Pilot", while the title of "Loin Like a Hunting Flame" was taken from the work of yet another contemporary, Dylan Thomas.