I didn't read Jeff's post, to which Brandon was responding, nor do I know to what film they are referring. But as a person who has committed myself to a religious order that eschews violence in all its forms, I feel compelled to butt in.
Depsite having attended Rochester Area Mennonite Fellowship for over ten years, I feel that every day I'm still learning what it means to be a Mennonite. So don't necessarily take my word as the word of all Mennonites, as it's filtered through my own experiences (before and up to joining) and personal understanding of pacifism.
First of all, pacifism in its strictest definition does indeed refer to refraining altogether from any kind of violence. I read a story about a Mennonite frontier family in America in the early 1700's who were slaughtered by invading Indians because the father refused to let his sons take up arms against them. At first glance, this might appear to be a complete waste of lives, and one could argue several compelling points against pacifism on this story alone. But what we don't often focus on is the impact such an act might have on others, namely the attacking Indians. Who knows what witness the family bore to that particular band of warriors on their journey back to their tribe? Who knows whether or not this act of determined principle didn't have ripple effects for years after? We don't, quite simply, so we act on the faith that what we do is right, even if we don't see the fruits of it.
All that said, my personal belief about pacifism includes not only refusing to act in violence (whether in word or deed), but to actively resist violence and speak out against it wherever it is being committed. It's quite the opposite of passive-ism. Christian Peacemaker Teams is an example of a wonderful organization that bears witness to violence between groups in strife-torn areas (Israel vs. Palestine, loggers vs Native Americans etc.) and reports globally about what is happening in those places. I went on a delegation to Palestine with CPT and was transformed by their approach to conflict resolution. There is a LOT that folks won't do when they think someone is watching.
Nonviolence is also about humanizing and deconstructing the aggressor. He is "us," not "them." When he is us, we reach out to help and heal rather than exact revenge. We respond with kindness instead of hatred, we forgive. Is this easy? Hell no! Is it above and beyond what a ordinary person should be expected to do? Hell yes! But is society going to change any other way? Violence begets violence begets violence begets violence. As the slogan goes, the only way to peace is peace. War only creates casualties and seeds beds of vengeance whose roots go down generations. The only way to stop this cycle is to stop committing acts of violence against each other, regardless of the reason.
So let's make it personal. Say (God forbid) someone commits an act of violence against my family. First of all, were I to say that I would have any idea how I would react in such a situation, I would be lying. Here's how I would hope to respond. If I were in any position to prevent the violence from happening--i.e. tackling a guy or putting him "in a headlock," I would certainly do so. I personally wouldn't consider this breaking any vow of nonviolence. Beating the crap out of the guy after I tackle him is a different story. I'm trying to prevent more violence from occurring while causing the least amount of damage to all parties involved. HOWEVER, I must insert that my first recourse would be to try to talk the guy down, etc. Speak to him as if he were human. Undercut his own effort to create an us vs. them dynamic--which is part of what enables him to commit violence against me and mine. I've heard amazing stories of women preventing themselves from being raped using this approach (not that I'm recommending that approach across the board, but we do need to reckon with these accounts). Hopefully we could calm the situation down peacefully in this way. But suppose (again, God forbid) things escalate beyond anyone's control and something terrible happens to my family. I hope that I would seek reconciliation and forgive. It's not something that happens overnight, but revenge doesn't undo the harm that's been done. And, like Joseph told his brothers, "You meant it to me for evil, but God intended it for good." And, as I mentioned earlier, there are ripple effects that we will never see. If my aggressor is not transformed, maybe someone watching my response will be.
I must reiterate that that last part is the weakest of all of what I said. The truth for me might be the complete opposite: in a rage, maybe I'd kill the guy. I really have no idea. I imagine, though, that it would wound me as much as, if not more than, dealing with the harm to my family. But it doesn't change what I believe to be right, and that is promoting peace wherever and however I can. It's the only way out.