Space Opera: Making Us Immortal with One Neat Trick

The simplest approach is to have your characters live a very long time. This is science fiction after all. The spice melange, central to the Dune saga can extend a human lifespan by hundreds of years, allowing the same characters to pop up throughout an extended story arc. Emperor Leto II, the God Emperor of Dune, doubles down by merging with the spice makers, which allowed him to live for more than 3500 years (with the unfortunate side-effect of turning into a giant worm, but you can’t conquer a galaxy without shedding a few legs).
The Dune books also employ another science fiction staple to keep its protagonists in the action—cloning. Fan favorite Duncan Idaho is the only character to appear in all six of the original novels. First seen as Swordmaster to the House Atreides (and a relatively minor character in the narrative), he is killed in Dune defending Paul Atreides and his mother from assassins. Fortunately, his dead body is collected and used to create a series of clones (called gholas) that play various roles of increasing importance in subsequent novels.
Frank Herbert—and his son Brian, and co-writer Kevin J. Anderson—also rely on family to keep the series going (in a couple different senses), tracing the tale of the Atreides family through many of the Dune books. Of course, family is a road to continuity not unique to SF. Alex Haley followed generations of Kunta Kinte’s descendants in Roots. In One Hundred Years of Solitude Gabriel Garcia Marquez spends the titular time with a family in the mythical town of Macondo. And Christopher Tolkien has certainly kept the denizens of Middle Earth from sailing off into the west.

Like most SF stories with faster-than-light travel, the Dune saga ignores relativistic effects. But for those authors obeying Einstein’s speed limit, time dilation is another way to keep characters in the tale. Joe Haldeman uses it to great effect in his classic SF novel Forever War, as does Orson Scott Card in the Ender’s Game books, to the point that the title character eventually meets his only slightly older sister while she’s an adult and he’s still a boy.
Alastair Reynold puts time dilation to good use in the Revelation Space series—as their name implies, his lighthugger ships travel very close to the speed of light, so the crew and passengers age much more slowly than characters in other parts of the story. This allows characters on the ships to impact events in the storyline hundreds or even thousands of apart. One of the Revelation Space stories, “Galactic North”, starts in 2303 CE but the protagonist, Captain Irravel Veda, continues her journey past 40,000 CE. Another Reynolds novel, House of Suns, combines cloning and time dilation to follow the myriad “shatterlings” of a single consciousness on never-ending adventures across space and time.

Another SF trope that ably extends a character’s shelf life is to put her on ice through cryopreservation, suspended animation, or hibernation of some sort. This can be a narrative necessity if starships can only travel a small fraction of the speed of light—otherwise, characters won’t make it to the end of an interstellar journey, much less to the end of the story. (Though sleeping through part of an opera is a pretty common event in the real world too.) Examples of cryosleep in sci-fi are too numerous to mention, but the movies seem to particularly favor it, perhaps because it provides a good excuse for neat special effects—and for beautiful actors and actresses to walk around in their underwear.
Another type of character that can wait around for a space opera to reach its climax: machines with artificial intelligence. This would include the sentient ships and space stations of Ann Leckie’s Imperial Radch novels (Ancillary Justice, Ancillary Sword, and Ancillary Mercy), whose memories stretch back thousands of years (come to think of it, Leckie puts the cryosleep trope to good use too).

The artificial Minds in the Culture series by Ian M. Banks can be as small as puppies or as large as continents, but they all operate on a timescale that dwarfs a human life, or human civilization at that. We could also include here humans whose consciousness is uploaded into some sort of machine, whether to be stored there (as seen in Black Mirror’s celebrated “San Junipero” episode, a story of love and digital immortality up for a Hugo this year) or downloaded into a new body (Richard K. Morgan’s Altered Carbon and sequels—and watch out for that one on Netflix in the near future)
Perhaps the ultimate way to wait for the mythical fat lady to sing in space opera is to seal yourself off from the rest of the universe. Whether inside the event horizon of a naturally occurring black hole or in an artificial construct, you can just hang out while time flies by outside. The Heechee in Fredrick Pohl’s Gateway novels, the Shrouders in Revelation Space, and a few humans in Liu Cixin’s Death’s End do just that. Coincidentally, all were hiding from a foe bent on wiping out every trace of advanced civilizations, so you can understand the need for a little quiet time.
So when you—or your clone, or the artificial reconstruction of your consciousness—have finished thawing out, or peeked out from your black hole, or been downloaded into a new shell, listen for that soprano. And hope your space opera has a happy ending rather than a Twilight of the Gods. And keep your peepers peeled for a machine civilization,  eager to clean up our organic mess.