Unlike in a regular campaign a dark campaign can be filled with much darker and off-putting content. As a player it is important that you communicate with the other players and your narrator what you are comfortable with and what is too much for you. Different people have different limits and it’s important that everyone at the table be aware of them. As a player you have a large flexibility in your actions. In a darker campaign things that would be inappropriate in a normal campaign are more allowed here. Torture and more grotesque actions may be available to you but you need to ensure that your character remains true to who they are. It can be tempting to use a character as a vehicle for you to experiment with limits but this will weaken the story in the long run.
Another thing to keep in mind is your character’s reactions to the events of a story. Most people are not well equipped to handle real horrors and the psychological ramifications of the events in a dark campaign affect the characters in various ways. Exploring the character’s reactions, coping mechanisms, and psychology can be as interesting as the events themselves. Sometimes it can even be more interesting. So when playing in a campaign with horrifying elements be sure to keep in mind how a real person would respond to these things and feel free to see where it leads you. And remember, it can be just as scarring to inflict horrors on others as it is to receive them. If your character goes down a dark path and begins committing atrocities feel free to explore that as well.
But the number one thing is to be respectful of the other players. Just because more messed up actions can be taken does not mean you have the liberty to do so with someone else’s character. Before any actions are taken against them be sure to talk to the narrator and other player about what they are comfortable with in regards to the other player characters. Dark campaigns can be really fun and very interesting but clear communication is a must to avoid offending or otherwise upsetting the other players and the narrator.