Appearances
The play is interested in the relationship between appearances and reality, and questions whether one's outward appearance is always reflective of their inner nature. It becomes clear quite early on in the play that DeFlores's ugly exterior is indicative of his wickedness, his appearance being an accurate reflection of his lack of morality. By contrast, Beatrice-Joanna is just as – if not more – evil than DeFlores, and by all accounts appears as an innocent, virginal, respectable noblewoman until the very end of the play. In the comic subplot, both Antonio and Fransciscus successfully dupe Alibius and Lollio into thinking they are madmen, showcasing how easy it is to change one's appearance and confuse others.
Deception
Related to the theme of appearances versus reality is that of deception, in which characters intentionally trick others into believing something about them. Antonio and Fransciscus are relatively low-stakes representatives of deception, as their plot ends comically. Beatrice-Joanna, by contrast, deceives Alsemero into thinking she is a virgin, while also deceiving all the other characters in the play (besides DeFlores) into thinking she is an upstanding woman. Her father even laments the fact that he could not see through her facade sooner and spare the lives of Alonzo and Diaphanta, casualties in Beatrice-Joanna's deceptive scheming.
Transactions and Commodities
Throughout The Changeling, characters are often motivated by a transactional approach to life, passion, and death. There is no better example of this type of mindset than DeFlores, who sees his murder of Alonzo as a service done as part of a transaction. Beatrice-Joanna wrongly assumes that DeFlores only wants monetary compensation for the murder and is surprised when DeFlores acts as though killing for money would make him a monster. Instead, DeFlores sees Beatrice-Joanna's virginity as part of the transaction, and convinces her to sleep with him in order to repay him. Similarly, Beatrice-Joanna promises Diaphanta that she will be paid handsomely for sleeping with Alsemero, suggesting that Beatrice-Joanna, too, sees women's bodies as commodities.
Gender and Sexuality
The Changeling is a unique play in that its central female character, Beatrice-Joanna, is also one of the central villains. One of the ways the play communicates Beatrice-Joanna's wicked nature (aside from her conspiring with DeFlores to murder Alonzo) is through the plot surrounding her virginity: Beatrice-Joanna is not only not a virgin, but also willingly has sex with DeFlores in the middle of the play, symbolically attaching herself to the character who is the more "obvious" villain. The play uses the concept of women's sexuality to imply that Beatrice-Joanna is unsavory, impure, and treacherous. By contrast, Diaphanta – who is proven to be a virgin based on her reaction to the "maid" potion - is innocent and pure, but is tragically sacrificed as a pawn in Beatrice-Joanna's larger scheme.
Sanity
The subplot of The Changeling takes place in an asylum, at this time referred to as a "madhouse." This plot features two noblemen from the castle, Antonio and Fransciscus, who dress up in disguise in order to present as "mad" and get closer to Alibius's wife, Isabella. By the end of the play, however, the madness of the subplot – which is generally lighthearted and silly – becomes a clear foil for the tragic chaos taking place in the castle. That is, the play proposes that "sanity" is relative and that appearances can be quite deceiving, ultimately suggesting that it is the characters in the castle – not the asylum – who are the most mentally unstable.
Passion and Morality
A number of characters in The Changeling are driven by their passions, often to the point of sacrificing their own sense of morality. Alsemero, for example, is so taken with Beatrice-Joanna that he extends his trip to court her, even though he knows she is betrothed to someone else. Franciscus and Antonio only disguise themselves as madmen in order to seduce Isabella. And Beatrice-Joanna schemes to have Alonzo killed in order to fulfill her desire to marry Alsemero. Finally, DeFlores kills two people in hopes of starting and maintaining his sexual relationship with Beatrice-Joanna. The play suggests that blind pursuit of one's passions is often destructive, and that passion as a motivation for action can often lead to tragedy.
Fate and Agency
The play is interested in exploring the relationship between fate – one's destiny that cannot be controlled – and individual agency, or choice. Of course, Beatrice-Joanna is the primary architect of the scheme to murder Alonzo, and acts out of her own agency when she conspires with DeFlores. The play suggests, however, that there is a deeper element of fate at work in Beatrice-Joanna's wicked behavior. When Beatrice-Joanna first encounters DeFlores, she berates him for his ugly appearance and weak personality, ultimately foreshadowing, ironically, her attraction to him as he transforms into a murderous maniac. In this way, the play suggests that Beatrice-Joanna and DeFlores were always fated to end up together given their complementary "ugliness" on the inside and out.
