A detective in other words is an investigator, who is involved in solving crime cases through different methods of investigation, such as suspect interrogation, canvass questioning, surveillance etc.
A detective may be a private person, also known as private investigator/ private eyes, or working for an agency, such as police, C.I.D., C.B.I. Detective looks for fingerprints or bodily fluids, and minutely investigates the crime location, carry out interrogation with the eyewitnesses to solve cases. Some detectives don't join an agency, but they work as private detective for the people willing to pay them to solve their personal matters. Also their contacts with other informants, which they develop over the years, help them to collect the evidence and can easily approach the potential suspect. Also to certain extent it is liable for detectives to mislead, lie, or give psychological pressure to the suspect but all these should go without violence and threat.