Ownership in shared fiction

I've a claim that I'd like to explore, and I'm explicitly looking for input from people who read this. The claim is about any kind of game which involves making up a story with other people. Here we go:

The importance you give to a character is the importance of that character to your imagined fiction. The more players differ in their investment in characters, the less they are sharing in the same fiction.

Firstly, to the rpg gamers (or ex-gamers) out there, does this strike you as a big deal? A player gets through a session where his character experiences real growth and does stuff that rewards her; in the same session another player flounders, doesn't really connect and ends up introducing elements inconsistent with his character. That this should be as big a deal as if the reverse was true (irrespective of how much the players care for each other)? That it hurts her story just as much either way - or else they're not making a story together?

To those with experience in drama: is there a mismatch between my statement and the way people portraying a role on stage or screen feel? I'm thinking through the implication of the fact that "what's my motivation?" is a cliche, whereas "what's her motivation?" is not. My thinking is that this may be a reality, but one that merely reflects the actors' focus on delivering their best performance. That when appreciating the value of the work as fiction, no-one values the quality of their own performance over anothers, and no-one values the quality of their lines over the lines for other characters.

Is this true? As an audience, I think certainly yes. As an actor enjoying his work as fiction, I say yes - but I'm not an actor. I should make it clear that I'm certain that from the point of view of personal satisfaction, or ambition, you're probably more preoccupied with your own performance more than others. I'm asking about the appraisal that is anchored in appreciation of the created fiction.

To those with experience in neither role-playing games nor acting, I'm almost interested in you most. What do you make of all this? Is it blase? Does it fit with your intuition of what you would be bothered about if someone said "let's make up a story together - i'll be rod and you be jane"? Putting aside the appreciation of the fiction, would you find it simply more comfortable to worry about one character entirely, and let the other person deal with the other - or does it make sense that if we're making a story, you'd welcome any input that was offered on what jane does next?

Ideas stimulated from many areas, but particularly here (and also here ).