A field interrogation is any situation in which a police officer asks questions, pertaining to a crime or a suspected crime, of a citizen prior to the time when the citizen is taken, by force or consent, to a police station for further processing.
Formal interrogation is to ask a person questions, sometimes to seek answers or information that the person questioned considers personal or discrete. Example: The captain interrogated the citizens on his whereabouts during the time the murder took place.
Broken down- (Formal interrogation is questioning someone on discrete information WITHOUT VIOLENCE)
(field interrogation is different, they arrest someone who they think is the suspect and take them to the police station for questioning.)
Hope this helps...:-)