On heroic characters

I've spent the past week revising a couple of scenes, double-checking the research aspects, and rewriting and rereading and rewriting again to make sure they're totally clear. In one of them, there is an explosion that my MC tries to shield his friends from. A writer friend who read the scene suggested that he not just shield them, but get hurt a little in the process. He was already being heroic, but bumping it up a bit so that we saw him take the consequences of his choice made him more heroic in that situation. Which makes me think of something someone from my local writers' group said recently. She'd been to a conference and one talk focused on things to make your MC more approachable. Among them were these suggestions:

How is your MC heroic?
How is s/he vulnerable?
What is s/he willing to sacrifice to get what s/he wants?

I'm starting a new book now and thinking about these things from the start. I really do love heroic, vulnerable characters who sacrifice something valuable for what they want most. But I think you have to be a bit careful with the sacrifices, otherwise, you're creating a character who just lies down and lets people walk all over her. We don't want to create a co-dependent character. We want someone strong, who's willing to risk it all for something they wisely know is worth more. They've got to be heroic so you know they aren't a doormat. And they've got to be vulnerable so they don't seem too perfect for the reader to identify with. These three things really have to balance each other for it to work.

I don't think heroics have to be huge. I think they can be small things the MC does without even thinking--things that show the measure of who they really are. Like Arthur in Kevin Crossley-Holland's books. He tries to be good at his own station, but he still helps Gatty catch the bull. He worries about the people on the estate who are treated unjustly. He bucks the system a little at a time, and the reader loves him for it. Sometimes very small details can give a reader a very positive impression.

Vulnerability is showing a character's flaws and fears and failures. It's what they stand to lose if their gambles fail. It's the part that makes them human. When a reader recognizes a shared fault or fear, the character feels human to them.

And sacrifice. You can't just give up all you are and have, just because it's your duty or because someone else said so. That's weak. You have to do it because you want to. Because you know it will hurt, but you're willing to fully accept those consequences in the hopes (but not perfect knowledge--it's still a faith thing) that it will cause something you want even more to come to pass. If you have given your character a chance to develop some heroic qualities prior to this point, they will be ready when the time comes to give all.

I'm still trying to figure this out. If you have any insights, feel free to share.