To help us is in this endeavor of greater holiness, Father Zuhlsdorf has a great post on participation in the sins of other people.
The entire post is worth a read, but he boils our participation down to nine areas:
1. Counsel: If you tell or advise another person to do something sinful, so that they do it, you have sinned by participation in that person’s sin.You might use this list to help when you next examine your conscience, which is wise to do each evening before falling asleep.
2. Command: If you have authority over another, and you forced that person to commit something which is sinful, while that person might have mitigated guilt, you don’t.
3. Consent: If you are asked if you think a sin is good thing to do, and have some power over the situation, and if you permit or approve or yield to the commission of the sin, you’ve sinned.
4. Provocation: You badger or drive or dare a person to do something such that he does it.
5. Praise of flattery: Pretty clear. This is another way of prompting a person.
6. Concealment: A person commits a sin and then you help that person conceal the evidence or the action.
7. Partaking: Another person is the principal person involved, but you are right there helping the actual sinful deed. For example, a person helping a doctor commit an abortion, a politician helping an aggressive governor or president or speaker of the house drive through recognition of contrary-to-nature “marriage” by providing a vote.
8. Silence: There is an old adage that “silent implies consent”. If a person with great authority or moral authority is in a position to stop a sin from happening, and yet stays silent and doesn’t get involved, then that may constitute participation in the sin committed. This is trickier to figure out, but it isn’t rocket science. There may be attendant mitigating circumstances, such as the probable invasion of Vatican City, the capture of the Roman Pontiff and destruction of the Church in many places. In the meanwhile one could work quietly. One cannot, however, do nothing.
9. Defense: Pretty clear. You defend or justify or give an apology in favor of the sin committed. This is not the same as what a defense lawyer does in the case of a person who is guilty.