That's a lot of fascinating questions. I'm not sure, though, that your initial empirical observation is valid. Sure, there have been many novelists with an interest in philosophy; but there have also been many philosophers with an interest in literature. You only have to look at Plato and Aristotle for clear examples.

Nevertheless, the relation between philosophical activity and literature generally, and the novel specifically, remains a matter for debate. Some interesting questions in this area are: what is it about literary types of language use that either can serve, or get in the way, of philosophy? Is the idea of a fictional world, narrative or character a useful resource for philosophy or, precisely because it is fictional, an irrelevance? And, in the reverse direction, what literary devices are already, and perhaps inevitably, at work in philosophical writing?

What is not very often asked, though, is the question you raise. Namely, whether philosophy completes itself in literature; that is, whether there is some problem or issue that originates within fairly conventional philosophical thinking but which can only be addressed adequately within literature. You mention the contextualisation of a philosophy in a world or character’s life. Let us suppose, for example, that there are certain universal features of human experience that are narrative in structure (personal identity, perhaps; or virtue). If so, then philosophy would have to take the idea of narrative seriously; and it might also be that the philosophy of virtue could communicate and explain itself most effectively through writing novels or plays. But this is still quite different from claiming that the philosophy of virtue could not be carried out except through novels or plays. That would take a great deal of extra argumentation!

When you consider that characters in stories are so much like us, it can be disconcerting: if they're like us, then we're like them, too. Indeed, what distinguishes us? Just that they're in stories and we're in reality? But couldn't they say the same about us?

That's a very tempting line of thought, but we should resist it.

There is a big difference between something's being represented as being so, and its being so. I can say that I have fixed the car, but that doesn't make it true that I've fixed the car. Someone might counter, yes it does--it makes it true according to you. But being true according to me is not a way of being true any more than being not true is a way of being true. This can be a little hard to see because of a very natural way we have of describing what someone has said. Often, instead of saying "Crimmins says that the car is fixed", we say, "The car is fixed, according to Crimmins". That makes it sound like the car is fixed, though not in reality but only in the according-to-Crimmins reality. But really it's just a picturesque way of saying the same thing, namely, that I have said something that represents the car as having been fixed.

The same is true in our talk about fictions. Instead of saying, "Conan Doyle, in the Holmes stories, represented that there was a detective named 'Holmes' who was very smart," we say "Holmes was a very smart detective, in Conan Doyle's Holmes stories," or even just "Holmes was a very smart detective". That makes it sound like there really was this detective, just not in reality. But, again, that's a mistake that arises from a picturesque style of describing what is represented (in this case, in fiction) as being so.

A feature of fictional characters that is puzzling if you think they are "real but not in reality" is called fictional incompleteness: did Holmes ever have a bunion on his left big toe? It's not just that we don't know---there's no fact of the matter, because all the "facts about Holmes" are settled by the stories, and it's unsettled there. This is no mystery at all if we realize that the only genuine facts are that the stories represent various things as being the case (but quite a mystery if Holmes is as real as we are).

Works of literature and works of philosophy are both the meaningful products of human thought and action. This makes them interpretable, which is an important characteristic of both. Moreover, both philosophy and literature are predominantly linguistic, although non-linguistic representations such as pictures and diagrams can play a part in either. In the contemporary context, both literature and philosophy are 'text-centric', but the centrality of texts is not a necessary part of either practice. Think of Socrates (who didn't produce any written texts) as well as traditions of oral literature.

Both literature and philosophy often address issues of deep human concern (e.g., serious ethical issues), and this is an important feature of both practices. But it is also plausible that this is a not a necessary condition of either. Philosophy doesn't have to address deep human concerns (e.g., you can philosophize about horror movies and--though I like them--I don' t think they're a matter of deep human concern), nor does literature. For example, a work of literature might involve the exploration of literary form and language rather than addressing any ethical issues.

We value works of philosophy and works of literature for many of the same reasons. For example, we praise both sorts of works for their originality, creativity and for their cognitive value. The last is particularly interesting. It does seem that we value both philosophy and literature for what we can learn (in some broad sense) from them.

But philosophy and literature also differ quite significantly. Philosophy is centrally concerned with truth and argument, with justification and the presentation of reasons. These are not central concerns of literature. It is plausible that fiction is the core of literature--there's literature outside the sphere of fiction, but fiction is the central case. And philosophy, although it may involve the use of fiction (in the Platonic dialogues or in the development of complicated thought experiments), is not centrally a matter of making fictions. Finally, even though cognitive value is important in both philosophy and literature, it seems much more important in philosophy.

I disagre with Professor Fosl's suggestion that philosophy is a sub-category of literature. The two categories overlap; that is, some works of philosophy are also works of literature. ( I think Nietzsche's Thus Spoke Zarathustra is a good case of this.) But there is a great deal of philosophy that is clearly not literature (e.g., most--perhaps all--philosophy articles in contemporary 'analytic' journals of philosophy). So I think it's not quite right to call philosophy a type of literature. There may be vague and overlapping borders between the categories, but there are clear cases on either side.