They Might Be Watching You
Sure, the FBI isn't wiretapping your phone, but they may be monitoring you in other ways. There are memes out there talking about the "FBI agent watching you through your webcam." Now, they're most likely not doing that, but they can monitor people via social media and search history. In fact, the FBI has found tons of evidence against people just because they put everything out there.
No "Good Cop/Bad Cop" Energy
Everything is dramatized for entertainment, including how negotiations and interrogations. In reality, FBI agents don't put on a "good cop/bad cop" schpeal. In fact, the agents are instructed to be cheerful and friendly in order to get the person they're talking to trust them. The most pleasant you are, the more information they may give up.