2) Don’t get bogged down in the rules
The finest films to feature time travel avoid detail about how it works. Take Terminator: when Kyle Reese is asked about the time machine, he snaps “I didn’t build the thing!”
You need some rules, sure. You need to keep it so that death means something and the adventures have a purpose. But don’t get too hung up on them, because…
…I have spent a lot of time thinking about this. I have read and watched an awful lot of fictional time travel, including a role-playing game system that tried very hard to nail down workable and watertight rules. I am an allegedly smart guy with a physics degree, and I have come to the conclusion that an entirely self-consistent system of time travel is an impossibility—at least, not without making a total nonsense about our ideas of narrative, identity and free will (the “multiple universe” interpretation). I smacked my forehead when I realised this because, duh, violating causality is the very definition of time travel.
Come up with something that works for your story, stick to it but don’t get too fancy or convoluted because if you do, it’s going to break down.
A friend has pointed out something else: whereas most sci-fi hardware has a starting point for your imagination, time travel does not. A space ship can be inspired by a sea ship or an aircraft; a laser gun is a gun that shoots a beam instead of bullets; a robot can be extrapolated from existing machinery. But a machine that moves you through the fourth dimension has no real-world ancestor. Therefore it can be as original, or as bizarre and anachronistic, as you want. Writers have played with this; from a phone in a microwave through hot tubs and, of course, the blue Police Box!
2b) Critics: you might not be as smart as you think you are
[Terminator 1-2 spoilers]
I have been pretty stupid myself, on occasion. I have done things like clicking on links for “10 of the worst movie plot holes….” although at least I usually remember to wipe my tracking cookies afterwards.
If I had a pound for every time someone has said something like “Ah, but John Connor could not have existed to send his father back in time to become his father!” then I could afford a pretty nice meal. (This sort of thing is called a “bootstrap future” by the way, after the proverbial boy who pulls himself out of the swamp by tugging on his own boot straps). In Terminator, the paradox was used correctly. It added interest and poignancy, but was not the main point or drama of the ending.
It’s time travel. It’s inconsistent by definition, and who knows how it would work if it was real? Accept it if you want to enjoy the film, but don’t accept half of it then moan about the other half. Unless you’re me.
2c) Put a bit of effort in what rules you do have, don’t let them be an Eleventh Commandment
[“The Flash” in various media spoilers]
So, The Flash yields to the temptation to go back in time and save his mother’s life. Only when he returns to the present, it’s different. In fact, it rapidly becomes clear that the entire world is rapidly going to hell. In the end, his mother has to die to save the world.
However, there is no plausible link as to why saving one woman’s life would cause Superman to be imprisoned, Bruce Wayne to die, Amazons and Atlanteans to embark on a globe-shattering war, etc. It’s all because our hero dared to tamper with the way things are Meant to Be. But this doesn’t happen when a villain jumps through time, oh no; they don’t accidentally make the world into a paradise. In fact, in some versions, it was a time-travelling villain who killed Flash’s mother in the first place.
This is a fairly common trope; someone makes a change in the past that seems positive, but it leads to something else bad or worse. It’s an interesting concept, and links in with the classic ends/means thing. It also, when done well, meshes with the reality of very few things ever being purely good or purely bad, of moral compromise, and the whole complexity of everything. But it can be done badly, and it can be done too often. It often seems as if the writer is applying a general “conservation of suffering” or at worst a divine “do not tamper with fate!” edict.
The first famous instance of a tiny change in the past causing massive change in the future is “The Sound of Thunder” by Ray Bradbury, from 1952. It has got a boost from chaos theory and its famous butterfly wing.
2d) It Is Written… Not!
A reversal of the above is where a character knows the future, but finds it completely impossible to change whatever they do. In some cases this is entirely plausible; how many people fail to change government policy, for example? But in others, it doesn’t seem that credible. ‘Oh dear, I’m going to bet on the wrong horse and lose my money? Well you can’t change the future, I suppose I had better get on and fill that betting slip out then.’
So what would happen if you tried to fill in the winning horse instead? Would your pen spontaneously combust? Would the ink magically re-arrange itself? Would the universe blow up or extra-temporal monsters appear to threaten you? There’s got to be some kind of mechanism that doesn’t make a nonsense of the character’s agency.