Having just reread LIGHTNING, one of my all-time favorite Dean Koontz novels, I was newly impressed by the way Koontz manipulates time travel to create suspense. It would be easy to think time travel lets characters do almost anything to solve their problems. But ingenious authors place conditions on time travel by which it complicates protagonists' lives and sometimes generates as much trouble as it prevents.
The most extreme example of time travel as more of a curse than a boon appears in THE TIME TRAVELER'S WIFE. If you've read the novel (or seen the movie), you'll remember that the protagonist jumps to different moments in his past or future (and in at least one case, after his own death) at random and involuntarily. He has no control over when or where he arrives. Worse, he can't take anything with him that isn't part of his body, not even tooth fillings. He can, however, appear in a time and place where he already exists and often does. In theory there could be any number of versions of him in a single location at once. Thanks to the nonlinear nature of his time travel, he can develop his relationship with his future wife in foreknowledge of their life together.
The TV series QUANTUM LEAP allows its hero, Sam, to leap only into the past and only within his own life span (although late in the series exceptions were finagled). He travels by changing places with a person native to that time period, who's made to wait in a sort of holding area until Sam leaps into another person's life. People in the other time see and hear him as the person he replaces. Therefore, he of course can't bring objects with him.
Connie Willis's time travel series (DOOMSDAY BOOK, TO SAY NOTHING OF THE DOG, BLACKOUT, and ALL CLEAR) features a research facility at mid-21st-century Oxford University, which sends historians back to study the past. They can go to any time and place, but some force inherent in the nature of time prevents them from getting too close to critical events, so they can't change pivotal turning points. (As far as they know—doubts on this issue plague the characters in BLACKOUT and ALL CLEAR.) They can wear and carry items from their own period into the past. But more than one version of an individual can't occupy the same moment. Although the exact outcome of accidentally doing so seems unknown, it's certain to be disastrous.
In Diana Gabaldon's OUTLANDER and its sequels, the magic inherent in certain spots on the earth (usually marked by stone circles) transports people into the past or forward to their own native era. The traveler can take as much as she can wear or carry. There's no control over when the traveler will arrive, though. Each circle seems to transport people a fixed number of years backward or forward. Therefore, nobody has the option of staying in the past for years and then returning to her "present" minutes after leaving. The same amount of time will have transpired as if she'd stayed in the present.
With the time turner in the Harry Potter series, a wizard can travel to any point in the past (or the future?—I don't remember that the books addressed that question), can carry objects along, and can exist in the same moment as an earlier or later version of himself or herself. The only limitation seems to be how long the device will allow someone to stay in a different time.
The closest approach I can recall to a time travel method that empowers the hero to do almost anything occurs in Heinlein's TIME ENOUGH FOR LOVE. Lazarus Long's time-traveling spaceship can go to any spatiotemporal location, and the system freely allows two or more versions of an individual or object to exist in the same moment. So any character can survive apparent death if a time traveler rescues him or her even at the last microsecond, as long as it's soon enough for transportation to the distant future where almost any disease or injury can be healed. This method saves "dead" people in both TIME ENOUGH FOR LOVE and TO SAIL BEYOND THE SUNSET. Maybe that power contributed to the impression of one reviewer who wrote about one of Heinlein's late novels that his characters "have no problems, only transient difficulties."
Stephen King devises an especially odd variant in his novel about the Kennedy assassination. The time portal in this book leads to a particular moment on a particular date in the 1950s, no other time or place. Most peculiarly, any trip back through the portal by anyone resets to the default timeline all changes made during the previous visit. The only way to ensure permanent changes would be to destroy the gate after the last trip. Thus, a traveler can always wipe out his mistakes (or someone else's) and start over. Unfortunately, all changes, whether bad or good, get obliterated. Each visit to the past is a total reset. (Or maybe not exactly, but that wrinkle shows up only toward the end of the story.)
The heroine of Koontz's LIGHTNING enjoys the protection of a mysterious "guardian" who pops in and out of her life at critical moments to head off various disasters. In fact, he has watched over her since before she was born, for his intervention prevents her from dying at birth along with her mother. Because he's a time traveler, he doesn't have to experience her life in linear order. If he learns an event has gone horribly wrong, he can return at an earlier date to fix it. (I won't mention what time period he originates from, since that's one of the book's thrilling surprises.) He can carry items back and forth. He wears a device that allows him to return to his own era at will, so he has no constraint on how long he can stay in a given period. Because of the limitations of his time machine, though, he's far from all-powerful. He can't travel to a moment when he already exists. Anyone who tries simply bounces back to the base site. Koontz uses this condition to create nerve-wracking suspense at the novel's climax. To save the heroine from certain death, the hero has to take advantage of a tiny window of opportunity. If that attempt fails, he has run out of moments he can travel to and still make a difference in the outcome.
For such conditions to incite the desired emotional responses in readers, of course, the rules have to be set up clearly in the early part of the work and adhered to with faithful consistency.
Margaret L. CarterCarter's Crypt