Eugenics
O'Neill explores the concept of eugenics through the Evanses' curse, which suggests that there are some negative traits that are passed down through families and thus necessitate selective breeding. People are basically animals, according to the theory, and therefore only the best, healthiest, and most intelligent of them ought to reproduce. O'Neill mostly discredits this idea through the fact that Gordon is neither Sam's child nor truly Darrell's–he is a quasi-reincarnation of Gordon Shaw.
Time
Nina says that there is no real present, only a past and a future. She laments the fact that the present is always miserable, always a time of nostalgia for what has already passed and a longing for the future with its promises of a better life. This is the "strange interlude" of the title, and she finally feels like she might have made her peace with it after Sam's death.
Queerness
Marsden is never explicitly identified as gay or bisexual, but O'Neill intended him to be the latter. Indeed, he is coded as homosexual in the way he dresses, comports himself, is obsessed with his mother, seems to dislike sex, etc. The other characters in the play make subtle comments about this fact as well, such as when Nina says he is a "fussy old woman" about his car.
Psychoanalytic Theory
O'Neill's use of the aside to probe his characters' thoughts, motivations, drives, and obsessions is a way to engage in psychoanalytic theory, which the Poetry Foundation defines as "a critical approach influenced by Sigmund Freud’s work on the unconscious and human behavior. Freud believed that the existence of three competing impulses in the psyche—the ego, id, and superego—and the conflict inherent in child-parent relations structured human responses to the world." All of the characters seem torn between their urges/passions and their moral compasses, and many of them have torturous parental relationships.
Sex
Many of the characters are motivated by sex, but it's in more complicated ways than might initially be conceived. Nina has a lot of sex, such as with the soldiers, but it is seen as sinful, a way to punish herself. Her sex with Sam is also something of a measure of absolution, and only with Darrell does she find some satisfaction. Marsden is obsessed with Nina but even though he constantly judges her sexual behavior, he himself does not seem to want to have sex with her. Darrell spends most of his adult life proclaiming he doesn't need the base act of sex, but then tumbles pell-mell into an affair with Nina. Sex rarely makes anyone happy, O'Neill suggests, but it's also usually the characters' own fault, not society's.
Happiness
The characters want to be happy. Nina wants to absolve herself of her past with Gordon and the soldiers and marry Sam and have his baby, then decides her happiness is with Darrell. Darrell decides sleeping with Nina will give him a modicum of happiness, an emotion he had decided earlier he didn't need. Sam finds happiness with Nina but mostly with material success. Marsden thinks happiness is getting Nina to see him as a viable partner. And Gordon Evans rather facilely sees happiness in being handsome, young, successful, and beloved. But their own ambitions, selfishness, and unconscious drives often get in their way, and by the end of the play it is likely that only the two most ignorant people—Sam and Gordon—can say that they were happy, and even then, O'Neill casts subtle aspersions on that sort of happiness.
Cycles
Edd Winfield Parks sees a governing theme of cycles in the text, writing that "Men and women are shells, acting a part, growing to other people, then away from them, but under and above all is life, taking care that new individuals shall be brought forth. Men and women cannot resist forces stronger than themselves, cannot tamper with the laws of nature; any attempt to do so results in tragedy. As the old people prepare to go off the stage, always there is the son of one couple, the daughter of another, to take their places. The cycle is complete, never broken, never ceasing."
