Cain and Abel -->As a follow-up to my short answer about keeping our fellow brothers from sin, "the dude" asked the following question:
I understand the obligation to correct others when necessary, but do I have an obligation to do it right away or can I wait for a good opportunity (even if it means the person continues in a sinful behavior in the meantime)? I am also wondering how far we have to go in correcting others. I know we don't have to go around preventing evil everywhere but where is the line drawn?A lot of this is a judgment call. I don't know if there are necessarily hard and fast rules regarding this sort of thing. I don't think anyone is asking you to be a super-hero. I think you just combat evil whenever you come across it: at school, work, home, anytime or place where you are interacting with people.
If you're in a conversation with a friend and he reveals to you that he likes to steal candy bars from the store down the street, then you tell him (in a way that will connect with him) that stealing isn't cool. If you see a person in Mass who lives an openly homosexual lifestyle but is receiving the Eucharist, you let him know as soon as you can that what he is doing is a grave sin. Do you tackle him in the middle of Church? I can't really answer that. Some would say that protecting the Eucharist trumps any sense of manners or reserve that we are supposed to uphold during the Mass. Others say that you wait until your earliest convenience so as not to draw attention to the spectacle and away from the Eucharist that everyone else is trying to participate in.
God doesn't necessarily give us a rule book for every particular situation in which a Christian may find himself. If we are trying to always inform our decisions by the teachings of the Church and if we are trying to be as faithful and well-intentioned as we possibly can be, then I don't know if anyone can ask anymore from you than that. You're trying your best, ya know? You're not a moral theologian. I'm not either. We're going to make mistakes
All I know is that sin is a very serious matter and that we should have a zeal for souls, for saving them and keeping them from sin. This means telling people things they don't always want to hear. This means approaching people you may not even know that well. Of course, there are ways to have conversations like this that aren't automatically offensive to people. You don't necessarily want to stand on a stool and start yelling at everyone. Use tact, and charity, and humility, and respect. Speak the truth in love. That's how we win people and give glory to God.
I hope that helps.