Isolation and Loneliness
A major theme in Interstellar is that of isolation and loneliness, and how they test our ability to think rationally and operate from an ethical standpoint as opposed to a mere survivalist one. Over the course of the film, some characters, like Dr. Mann, abandon everything they stand for in order to stay alive. Others become more animalistic and revert back to instinctual needs, as when Romilly and Cooper listen to the sound of rainfall and crickets to help them cope with their distance from Earth.
Another example of these themes is showcased by characters who feel alone even while surrounded by others: Cooper, for example, is a NASA pilot, not a farmer, and so he feels out of place as a farmer helping his community and raising his kids alone. When he's offered the opportunity to venture into space, he can't help but chomp at the bit, and through this we see his longing to be back in the much bigger fields for which he feels he was destined.
As a third example of isolation and loneliness, Professor Brand holds on to the secret that Plan A is unfeasible and that Cooper's mission will not save the lives of those on Earth. We see that he pays a great price for keeping this secret in the scene where he admits it to Murph on his death bed, shattering her. Interstellar shows us that whether or not we can withstand isolation and loneliness depends on how strongly we have been tested by its confines. No one can really know their own limits until they have been there as well.
Love and Human Connection
Both love and human connection more broadly are significant themes of Interstellar. Dr. Brand is the first to say explicitly that love, as a force, is capable of transcending time and space, a notion that will return to prominence when Cooper finds himself in the tesseract, where his love for Murph allows him to connect with her and help her solve the problem of gravity. Brand finds the scientific, mission-driven part of her brain conflicting with her desire to see Wolf Edmunds again, a fact that Cooper cites as a reason to discredit her. Importantly, Brand argues that the pull she feels toward Edmunds is not a reason to count her arguments out, but rather to take them seriously, as she believes that it's possible for love to be quantified and affect outcomes as much any other data. Only once Cooper finds himself connecting with Murph inside the tesseract does he come to agree with her.
Endurance
The Earth in Interstellar is dying; crops are becoming extinct and food supplies running short. These elements test the ability of each character to endure and survive. Cooper must endure the heartbreak of leaving his family to go in to space; Murph has to endure the anger she feels towards her father for leaving and still believe that he will come back; Professor Brand must endure the price of keeping his enormous secret; the people of the world must endure the dust storms and find a way to make for food.
It's no coincidence that the space station that carries our heroes into the cosmos to search for new habitable worlds is called the Endurance. The station is a symbol of perseverance, of surviving impossible odds with the hope that tomorrow will bring clearer skies. Professor Brand often reads the poem, "Do Not Go Gentle," as narration over scenes of the Endurance's perilous journey through space. The poem itself is an anthem of endurance, of raging against death, one that ties perfectly into the station's purpose and the larger goal of the astronauts on board.
Sustainability
Interstellar explores the concept of sustainability, both of resources and of the human race itself. On Earth, Cooper and the other farmers fight to sustain the growth of crops like corn and okra, and in turn ensure their own survival. When NASA deems this impossible, they look next to the sustainability of life on other planets (e.g. the presence of water and possible marine life on Miller's planet). While aboard the Endurance, they must ensure their ability to survive through long-term measures like hibernation chambers, as well as food and fuel sources, to sustain themselves until they can reach a new planet. The themes of sustenance and sustainability are present in every chapter of the film's perilous ride.
The Human Capacity For Evil
Aboard the Endurance, Dr. Brand and Cooper discuss how nature can be formidable and dangerous, but never inherently malicious, as the universe has no understanding of right and wrong. They conclude that that's what they're bringing out into space by traveling through the wormhole, and once Dr. Mann betrays them, this is shown to be true. The fact that Dr. Mann's planet is cold, stark, and uninhabitable is lamentable, certainly, but not personal; it didn't happen out of spite. Nevertheless, Mann is driven to spiteful actions to save himself and doom his fellow humans, a display of evil aggression that he himself acknowledges is cowardly. Dr. Mann represents the human ability to be intentionally malicious, something that he had to introduce to an otherwise indifferent universe beyond our solar system.
The Cycle of Denial
While talking with Donald on his front porch, Cooper muses about the futility of the farmers' efforts to grow enough food to keep humanity alive. He implies that they're kidding themselves by telling themselves that next year will be better when they know deep down that it won't. His understanding of this is part of what convinces him to leave his farm behind to find humanity a new home. Tom's actions later in the film, however, become a depressingly familiar reflection of this denial. Despite the loss of his first son and the worsening heath of his wife and second son, Tom refuses to get them medical treatment, instead focusing on growing more and more crops despite the grim results of his efforts. These actions, as well as his proposal to try farming the land of his dead neighbor, is a direct reference to Cooper's earlier statements about the farmers' inability to admit that next year won't be better.
Sacrifice
The difficult theme of sacrifice is a component of Interstellar's plot. There are many instances in which characters either deem themselves or are deemed by circumstance to be necessary sacrifices for the greater goal of completing their mission. One of the first such instances of this comes in the form of the Lazarus pilots who venture through the wormhole with the understanding that most (if not all) of them will die before they can be found, especially if their planet is not a viable candidate for harboring life. On Miller's planet, Doyle's sacrifice becomes an unfortunate necessity to save both Brand and TARS, and later, Dr. Mann's accidental sacrifice eliminates his malice and cowardice from ruining the mission. Finally, sacrificing TARS and then Cooper to Gargantua becomes the only way to acquire the data necessary to solve the gravity equation, save humanity, and simultaneously send Brand onward to Edmunds' planet. From a screenwriter's point of view, motivating sacrifices properly is a matter of understanding the goals of the story and each characters' role within the plot to achieve those goals. TARS and Cooper's roles became nothing more than weight that the Endurance needed to shed, and so they were forced into the black hole with the hope that Brand would make it to Edmunds' planet and that they could transfer the quantum data they found inside Gargantua before they were killed.
