Cameron became more concerned with being a role model than he was with his role because he had found a spiritual home in a branch of Christianity that had an almost entirely negative concept of virtue. According to this form of religion, being good means not doing certain things -- not doing a lot of things, actually. And being really good, I suppose, means doing almost nothing.
According to this view, being morally good doesn't take any work. It's not something you have to learn, or study, or practice. It comes by fiat, through God's intervening grace. We saw this idea of moral goodness in the book Left Behind: say the magic words and God will transform you into a good person.
Clark goes onto to explain how this belief led Cameron to think that was no need to work to improve his acting skills, but there's another point to be made here. As Cameron explains in his own words, he became concerned with the idea of being a good role model. But as his colleagues on the show explain, he withdrew from them. Cameron wanted to be a positive role model, but he passed up a chance to be a role model to the very people he worked around and knew the best. Instead, he seemed to want little to do with them, presumably because he felt he wasn't like them anymore.
Now, this is a phenomenon that I've personally witnessed. To be fair, I understand the motivation to pull away, at least in some sense. As anyone who has ever tried to turn their life around can tell you, sometimes you have to get away from the people who are part of that old life and who would-intentionally or unintentionally-drag you back into that life if you gave them a chance. But that's not always the convert's motivation (and here, we can be referring to a recent convert of any religion or particularly strong belief system.) To one degree or another, that convert is willing to dismiss their friends and family who don't convert with them from their lives. Maybe they're afraid of being given a hard time about their beliefs, maybe they're afraid of a weakening of their beliefs after having their beliefs challenged. Maybe they're no longer interested in people who don't believe what they do, or share their strong feelings of joy. Maybe they think it's wrong to associate with "sinful" people. Maybe they think they're better than those people now. But whatever the reason, people who follow this route make a deliberate decision to exclude those who don't believe what they do, no matter how long or how well they know them, and embrace only those who do. This is another iteration of this "negative concept of virtue" that Clark discusses, an exclusion of those to one degree or another who don't share the same strong beliefs. Instead of desiring to share their beliefs, or live comfortably in their beliefs around those who don't share them, this particular type of convert runs away to the comfort of those who think like he or she does.
In this sense also then, being good doesn't require any work. Or at least, not as much work as it might to continue being a good colleague, good friend, or good family member. And it sends a terrible message to those who knew this person; that Christianity is exclusionary, that Christians are not open to others, that Christians are fearful of the larger world and prefer to retreat from it than attempt to change it personally in their lives, that Christians value the words of those they've only recently met over the words of those who have known them and cared for them the longest. Which, one would think, is pretty much the exact opposite of the message the evangelical Christian hopes to send to the unconverted.
Now obviously, most Christians are not like this. And after an initial conversion, an evangelical Christian may return to the social circles he or she once roamed in, more comfortable in his or her beliefs or more willing to attempt to bring others on board with his or her words and deeds. I've experienced that outcome personally as well. But it was years in the making.
I hardly know enough about theology to make some argument about what being a "good" Christian does or doesn't entail. But I do know those who are most likely to make a positive impact, those who are most likely to change the world, are those who are not afraid the world and the people in it who aren't like them. And I would imagine that no beliefs are stronger than those that have been put to the test and survived. It's unfortunate that many who believe strongly that they have a positive message to share with the world, don't feel the same.