A time to be silent and a time to speak

What should we do when we see a non-Christian friend of ours doing something we think is wrong? It depends a bit on where they're at - whether they would be glad of our advice or whether they are opposed to what we think's right. In either case, it might be nice to say something in the hope that it will help them out. Or so that they might see that what they're doing is not on - in the hope that, at the right time, they will realise their need for forgiveness. Or it may be better to keep our mouth shut to avoid implying that if they would only act this way it would make them a good Christian like us, and that if they refuse we will be judging them. Whatever it is we decide to do, we need to be careful not to become self-righteous. It's not like we're these perfect beings - and it's only because of the Son that we can have confidence before the Father. We need to keep imitating the surprising mercy, patience and graciousness that he has shown to us sinners.