Structure of the novel
Something Wicked This Way Comes can be interpreted as an allegory of the struggle between good and evil, with the human characters Will, Jim, and Charles on the side of morality, and Mr. Dark and his carnival on the side of sin and temptation. As in many other fictional works revolving around the same concept, good prevails in the end, not with supernatural or physical powers, but with purity of heart. Jim represents good that is always on the verge of giving into temptation, while Will, though he has crises and doubts, is the part of us that resists giving in.
As in Dandelion Wine, Bradbury infuses the novel with nostalgia for his childhood. However, Dandelion Wine embodies the idyllic memories of youth, whereas Something Wicked This Way Comes superimposes folk-tale and supernatural elements over a small-town Americana setting in order to explore the dark undercurrents that surround the transition to adulthood.
Transition from childhood to adulthood
The carnival's main allure to its participants is its ability to change age easily against natural causes. Jim wants to become an adult by riding the carousel forward, and Charles Halloway initially considers riding the carousel backwards. Even Will is somewhat tempted by the offer for a free trip to adulthood.
Charles, however, quickly sees that a ride on the carousel can have unforeseen circumstances, because despite age being changed instantly, the carousel would not change the mind. "If I made you twenty-five tomorrow, Jim, your thoughts would still be boy thoughts, and it'd show! Or if they turned me into a boy of ten this instant, my brain would still be fifty and that boy would act funnier and older and weirder than any boy ever."
Because of this, a person who rode the carousel would be reformed only physically, with the same sins and emotions contained inside. Moreover, his new physical form, created unnaturally, would alienate him from his family and peers, leaving him with nowhere to turn except for the carnival.
Charles best personifies this theme; while he is middle-aged in body, he is still youthful in mind and spirit. At first, he sees the two conflicting personas within him as irreconcilable and longs to be physically young too, but his active participation in toppling the carnival proves to him that mental fitness and perception of one's age is more important than physical health.
Ironically, Will and Jim can be said to have aged prematurely in the novel; the horrors of the carnival force them to grow up fast to be able to deal with its tricks on a knowledgeable level. Furthermore, Will and Jim do take a brief ride on the carousel before Will pulls Jim off, and they are never shown reversing this process before Charles destroys its machinations. Thus, it can be stated that they, in fact, grow up slightly. In this case, though, Will and Jim have also matured emotionally, too, having had their first encounter with evil. This enables them to grow more proportionally in both physical and emotional status.
Belief and fear
The novel also conveys the theme that the power of people, objects, and ideas have over you depends on the power you instill in them with your own mind. Because of this, the carnival is able to easily take advantage of the common human fears of aging, death, and loneliness which everyone has or relates to.
Charles Halloway is the character who learns the most about this; he initially views death as unpleasant and it thus becomes a sinister force to him that the Mirror Maze magnifies. However, Will's words of love help him to see that age does not matter if one focuses instead on the knowledge and affections gained with it, and as his fear vanishes, so does the Mirror Maze. He also is able to defeat the Dust Witch once he realizes that she does not have ultimate control over him. With his belief in her powers gone, he turns the tables on the Witch by instilling the same fear in her of his smile that he used to have of her magic.
Viewpoint on life
Self-centered desires and wishes are portrayed as the base of human malice and unhappiness because they blind people to the blessings of life with an unattainable dream. The novel's main example of this is Miss Foley's seduction by Cooger's promise of youth that causes her to fail to see his deception as her "nephew," and lose her rightful place in society.
It is implied that the counter-force against this is acceptance of one's faults and an enthusiastic pursuit of the everyday joys of life, signified by Charles' simultaneous running with Jim and Will at the end of the novel. The fact that he is nearly forty years older than them pales in comparison to the pleasure he gains from simple human companionship.