With regard to the kind of indie RPG that is unsuited to long 'campaigns' - think of My Life with Master, The Mountain Witch or Polaris - I have often heard people say that this does not appeal to them, because they need several sessions in order to 'get into' their character, and thus long campaigns to fully enjoy roleplaying him or her. Every time I heard this, I thought of the cardboard characters I had played in my longest games and the powerful, deep characters I had played in short, narrativist indie games, and I dismissed these complaints. This was foolish. Instead, I should have wondered whether there are not different types of characterisation, different ways to give a character what I might call 'psychological depth'; and different playing styles and games that allow us to create this kind of depth.
I will now present four types of psychological depth. This typology is probably not perfect; it is almost certainly not complete. But perhaps it can serve as a starting point for discussion.
A. Choice made difficult by internal tension
The character must make a choice between two or more options all of which she would like to make, but because of different kinds of reason. For instance, a mother who loves both her honour and her children must choose between the two (Medea); a boy must choose between fighting for his country or caring for his old mother (Sartre's famous example). We get to learn the character because we see her in a situation in which she must show which of her important drives is the strongest one. We get to learn the character because we see her in a situation in which she must decide who she really is.
This kind of psychological depth is exactly what the majority of current narrativist designs give us. Sorcerer, Dogs in the Vineyard, The Mountain Witch: all of them are designed to provoke this kind of choice. It is especially suited to shorter games, as the tensions within the character tend to resolve themselves. (But think of The Shadow of Yesterday: its system of buying-off and buying keys is a way of resolving and setting of new tensions.) Play is often dramatic; the characters larger than life, 'literary' characters, bigger and sharper versions of ourselves. We may feel for them, but we could not be them, and we probably do not love them.
B. Changes through external experiences
The character experiences things that cannot possibly leave her unchanged. A child watches her mother die? A curious librarian researching a bizarre cult almost comes face to face with Cthulhu himself and goes half mad. We get to learn the character not because of the choices the character makes, but because of learning of the powerful experiences she has had. We now know the person as 'the girl who watched her mother die', and will understand everything else she does in the light of this knowledge of her psyche. By knowing her history, we understand the current workings of her mind.
This type of psychological depth would be catered to by games involving (1) relatively long campaigns, and (2) a system that somehow encourages players to play their characters with a regard to previous experience. A game like Call of Cthulhu, with its sanity statistic, comes into mind, although it is relatively one-dimensional in this respect. One could use the fallout-techniques of Dogs in the Vineyard to achieve something much like this. Note how this type of psychological depth is not facilitated by The Shadow of Yesterday, where keys vanish without a trace.
C. Moments of openness
The character open her soul to you, as it were, in a moment of friendship, love and therefore vulnerability. It is the RPG equivalent of a late night conversation with a good friend, in which you speak of fears, hopes and desires that remained hidden for years. We get to learn the character because she drops the walls that always guard adult personalities, and allows us a chance to see her as she really is. We get to learn the character at the very moment she makes it possible for us to love her with all her faults and weaknesses (all love implies the possibility of pain).
The game that immediately comes to mind is Breaking the Ice, with its brilliant mechanics that force the characters into revealing their vulnerability, thus becoming more like us than any epic character could ever be. It is also the kind of psychological depth that my playtests make me believe Shades can offer its players. My Life with Master is an interesting case: it has strong elements of A and C and combines them by making the conflicting forces in the minion's psyche all types of vulnerability (Weariness, Self-Loathing, Love).
D. Lengthy observation
The character slowly reveals herself to us because we observe her for a long time. She makes no particularly revealing choices; she has no harrowing experiences that scar her forever after; and she does not open herself to us in a moment of love and vulnerability. Instead, we just get to know her by seeing how she reacts in many different situations; and although we may feel that we do not really know her inner thoughts and counsels, there is nevertheless an important sense in which we know her well. Make no mistake: this is the way we know most people in real life.
Interestingly, designing for this kind of psychological depth involves the following two things: (1) the system should facilitate long games, (2) the system should not facilitate A, B or C too much. Why not? Well - A, B and C will tend to interfere with D, by tempting us to interpret the characters actions in the light of the insights they have given us. That doesn't mean that D can't be combined with A, B or C, but the others need to be restrained in order to let this one flourish.
This kind of psychological depth, then, might be most easily achievable by games such as Dungeons and Dragons, GURPS, World of Darkness, and Das Schwarze Auge, all of which offer long games without too much focus on existential choices, life-changing experiences or moments of openness. I find this counterintuitive, but pleasing.