I can't remember the legal term for hmm... not knowing details about the crime. I will explain on the example:
Let's say that we suspect that our friend is going to rob a bank. And he asks us "do you want to know what I'm going to do tonight?" And then we say 'no' because we want to be able to honestly tell the police "I don't know anything about it."
What's the legal term for that? It's pretty common in the movies and TV shows.