In ‘The Unquiet Dead’ Rose ponders how she could die in the past, before she is born. This illustrates the difficulty that some people have with the concept of time travel. This is Rose’s first trip into the past and she is obviously struggling with the concept of what is happening.
Does she imagine that this akin to ‘A Christmas Carol’ and she is not actually there? While she can sense the 19th century and interact with those around her does she believe that she is just a projection and that her physical body is still in the 21st century?
She also suggests that her objection is based on the illogic of her dying before she is born. Does she believe that ‘time’ or some other entity just won’t permit this confusing state of affairs?
To her credit Rose doesn’t act as if she believes she is invulnerable prior to this scene. She is probably just trying to rationalise her way out of the situation, trying to find a logical explanation for how she will get out of this unharmed.
It could be amusing for a player character, new to time travel, to have this belief. They might be incredibly bold and risk great danger because they think they can’t be hurt. They are in for a nasty shock when they are injured for the first time.
The Doctor’s explanation that time can be twisted into new shapes is similar to the 10th Doctor’s discussion in ‘Blink’ that time isn’t strictly a linear series of events. Time in this context is Roses’ life. In particular that the date of her birth and death don’t necessarily have to occur in that order.
Within the Doctor Who universe time is often treated as a ‘place’. The TARDIS can remove people from one location, whether that be a planet or time period, and deposit them in another. Here time is more of an abstract concept.
Part of how time is presented is that the viewer follows the main characters in their travels, thus we have a sense of continuity. They might be going backwards and forwards in time but we still know in which order those adventures happen to our heroes.
An outsider would have different perspective here and this might be what Rose is referring to. Anyone observing or recording those events is going to have difficult time explaining how Rose dies before her birth, especially if they aren’t aware of the TARDIS or what it does.
This can be seen in discussions of the time travel film ‘Primer’ where people will often refer to the time machine producing ‘duplicates’. This isn’t what the machine is doing. It isn’t creating people but transporting the same person up and down the time stream. Yet because these viewers perceive there to be two (or more) of the same person then one must be the original and the other duplicates. A similar discussion occurs in the Spanish film ‘Time Crimes’.
Perceiving time travel in a different way, especially from the point of view not familiar with the mechanics can inform our knowledge of how NPCs will act and behave. Their observations may be terribly wrong but they will still act on those conclusions.
Adventures can be developed based around how people react to incidents of time travel they’ve experienced or learn of. This could be a good way to have sequel to the PCs or the Doctor’s adventures, with people trying to puzzle out what happened and reacting accordingly.
Mysteries can also be developed for PCs to investigate that turn out to be time travel. This can be particularly effective if your campaign focuses on those with little to no experience with it, such as a UNIT or Torchwood campaign.
While we know Rose is wrong in her belief that she can’t die what if she was right? This removes the element of physical danger to a campaign, with PCs unable to be die before the date of their birth or after the date of their death.
This can help encourage players to send their characters into dangerous situations. It also adds to their mystic, as this will make them appear inhuman to others. Their enemies will fear these unstoppable beings who appear from nowhere to stop them.
You may wish them to demonstrate other evidence of their disconnection from the current time. It could be that they are also immune to extremes of temperature, unable to feel the cold or heat. They might not need food or water nor require rest.
This could be how the Time Lords acquired their reputation for being god-like beings. In their native time period they are the ordinary beings we see in episodes set on Gallifrey but when they travel to other eras they can not be harmed or stopped.
If this is how time travel works them travellers would have a section of time, the duration of their life, which they must avoid or be made vulnerable. Mysterious beings like the Doctor could have this period in the distant past or future, while for most companions this is the 20th or 21st century.
There are still opportunities for peril. Time travellers might not have any great physical strength and so risk being captured. Such travellers could be imprisoned indefinitely, especially if they don’t require sustenance.
If the time travellers are projections then their presence in other time periods could require concentration. Anything that frightens them or causes them to believe that they would die can cause them to return to their own time period.
You could reduce the PCs ability to interact with the past or future. It could be that they aren’t actually there and so can only observe. If they are able to interact it isn’t real and when they return to their own time period they find that any changes they made aren’t reflected there (this could mean that changes create divergent timelines).
Within your own campaign you are free to decide how time travel will work. Equally PCs and NPCs can come up with their own interpretation. Scenes like this from ‘The Unquiet Dead’ can allow characters to discuss what they think is happening.