Under American law this is probably the worst possible advice you could give someone. Why? Because if you end up in front of a jury, that police officer is go

Under American law this is probably the worst possible advice you could give someone. Why? Because if you end up in front of a jury, that police officer is going to testify to that jury that you said "you'd tell them anything when they granted you immunity", a truthful statement, and the jury is going to fill in whatever gaps the prosecutor suggests they fill in. If you are in police custody you ask for nothing from the police. You tell them that you are remaining silent until your lawyer arrives. And then you shut up.