Allusions to other works
As in other novels, certain characters cross over from other stories, making cameo appearances, connecting the discrete novels to a greater opus. Fictional novelist Kilgore Trout, often an important character in other Vonnegut novels, in Slaughterhouse-Five is a social commentator and a friend to Billy Pilgrim. In one case, he is the only non-optometrist at a party, therefore, he is the odd-man-out. He ridicules everything the Ideal American Family holds true, such as Heaven, Hell and Sin. In Trout's opinion, people do not know if the things they do turn out to be good or bad, and if they turn out to be bad, they go to Hell, where "the burning never stops hurting".
Other crossover characters are Eliot Rosewater, from God Bless You, Mr. Rosewater, Howard W. Campbell, Jr., from Mother Night and Bertram Copeland Rumfoord, relative of Winston Niles Rumfoord, from The Sirens of Titan. Mr Rosewater says that Fyodor Dostoevsky's novel, The Brothers Karamazov, contains "everything there was to know about life". Vonnegut references The Marriage of Heaven and Hell at one point, when talking about William Blake, Billy's hospital mate's favourite poet.
It should be noted that while Vonnegut re-uses characters, the characters are frequently rebooted and do not necessarily maintain the same biographical details from appearance to appearance. Kilgore Trout in particular is palpably a different person (although with distinct, consistent character traits) in each of his appearances in Vonnegut's work.
In the Twayne's United States Authors series volume on Kurt Vonnegut, about the protagonist's name, Stanley Schatt says:
By naming the unheroic hero Billy Pilgrim, Vonnegut contrasts John Bunyan's "Pilgrim's Progress" with Billy's story. As Wilfrid Sheed has pointed out, Billy's solution to the problems of the modern world is to "invent a heaven, out of 20th century materials, where Good Technology triumphs over Bad Technology. His scripture is Science Fiction, Man's last, good fantasy".
Allusions — historic, geographic, scientific
Slaughterhouse-Five speaks of the fire-bombing of Dresden in World War II, and refers to the Battle of the Bulge, the Vietnam War, and the civil rights protests in American cities during the 1960s. Billy's wife, Valencia, wears a Reagan for President! bumper sticker on her car, referring to Reagan's failed 1968 Republican presidential nomination campaign. The bumper sticker was edited out of a broadcast version of the film which aired on at least one cable channel during or after the Reagan administration. Another bumper sticker is mentioned that says "Impeach Earl Warren."
The slaughterhouse in which Billy Pilgrim and the other POWs are kept is also a real building in Dresden. Vonnegut was beaten and imprisoned in this building during World War II and it is because of the meat locker in the building's basement that he (and Billy) survived the fire-bombing; the site is largely intact and protected. One can visit it and take a two-hour guided tour called "Kurt Vonnegut Tour".