Empty Cop Threat

In cop shows, when the good guys try to get information out of witnesses, the witnesses often don't want to tell the cops what they know, sometimes for real reasons, other times for no apparent reason. When this happens, the cop often threatens to charge the witness with obstruction for not coming forward with whatever information they want from them.

The witness is always shocked and scared by this because, in TV land, people on cop shows always believe that they can be arrested and thrown in jail for not telling the cop what they want to hear. (Though you can kinda forgive them for thinking that in a fictional universe where Stop or I Will Shoot is prevalent.)

In Real Life, obstruction of justice is only applied in the most blatant cases, when the witness is found later to actually have something to do with the crime (and has failed to plead the Fifth Amendment or local equivalent), or when the prosecutor who gets the case is really, really frustrated. Charges may be laid when it is discovered that a person questioned in an investigation, who is not a suspect, has lied to the investigating officers. You can be called as a witness and be forced to testify or be held in contempt of court.

But only in major cases would the court bother and it would be up to the prosecuting attorney to decide this, not the police. No prosecutor is going to waste their time on someone solely because the cops complain they are uncooperative and might have witnessed something. Both the police and prosecutors know many witnesses will just lie and say they saw nothing unless the authorities have real evidence.

In most common law jurisdictions, most of the time, the right to remain silent allows any person who is questioned by police merely to refuse to answer questions posed by an investigator without giving any reason for doing so.

Note that there may be some Truth in Television in this. Just watch any police reality show.

Sometimes a prelude to the Jack Bauer Interrogation Technique. Automatically assumed in the event of a Dramatic Gun Cock or High Altitude Interrogation.

Comic Books

 * Subverted in the Ed Brubaker series Criminal, where an investigating officer shows a photo of his target to the proprietor of the local Bad Guy Bar and threatens "obstruction of justice" if he doesn't identify him. The bartender just smiles and tells him he's more than welcome to try and press charges on a 60 year old man for not recognising a particular face in a dark and crowded building.
 * A superheroic equivalent is Batman's tendency to hold suspects by the foot over the edge of a building until they talk. Any criminal who knows the first thing about Batman would know that Batman would never actually drop anyone off a building, but it always works anyway.
 * To be fair, it's one thing to be confident with both feet on the ground, another to be confident dangling from a building.
 * And anyone who knows the first thing about Batman knows he's Crazy Prepared, so he likely has something in mind in case the subject refuses to talk anyway.
 * This was pretty heavily subverted in The Dark Knight when he does drop a mobster from a building, as it was high enough to break bones but not kill.
 * In an episode of Justice League, the Flash pulls this stunt on a mobster as he's looking for Shade. The guy calls the Flash on his bluff, saying that he can't pull off what Batman can... until he does drop him.
 * In the same show, this fails spectacularly when Superman tries to intimidate Copperhead for information concerning an assassination attempt on Aquaman.

Film

 * Averted pretty hard in the beginning of Sin City. Junior Rourke believes that John Hartigan is showing up to arrest him. He gloats that there is nothing Hartigan can do. He probably didn't expect to get his nuts blown off.

Literature

 * In the novel Black House, retired detective Jack Sawyer uses the Obstruction of Justice tactic on a group of boys being questioned on the whereabouts of their friend. It's made obvious to the reader that this is merely an attempt to impress the boys, as they are being silent about what they know because they're afraid they might be blamed for the friend's disappearance.

Live Action TV
"Dean: Withholding information from the police is a capital offense. (The uncooperative witness doesn't really buy the threat. Sam clears his throat, with the meaning "Quit being an idiot, Dean.") Dean Uh, in some parts of the world, I'm sure."
 * Law and Order is probably the worst offender. Any individual is threatened with Obstruction of Justice, and any business is threatened with a mob of NYPD officers searching everything and going through all their records (as if the NYPD has nothing better to do). People fold incredibly fast under these threats, because the show doesn't have the time to allow each witness to stonewall or play dumb. If one actually does hold out, it is the script equivalent of the Unmotivated Close Up: That specific witness has a critical piece of information.
 * On the same show, a variation of the Empty Cop Threat comes from the prosecutors' frequent offers to "take the death penalty off the table" in exchange for information. New York State hasn't executed a prisoner since the 1960s, and the death penalty was declared unconstitutional by the state's highest court in 2004.
 * A 2006 Law and Order episode had McCoy attempting to appeal that ruling so he could get the death penalty for a man who had shot four grammar school students after killing a guard in his escape on the way to the courthouse for an appeal hearing of his earlier conviction for killing four people in a restaurant. The ruling is not overturned, but, in the end, the man is killed himself by a victim's father. McCoy thought he deserved it after nine (known) murders.
 * Cops (series).
 * Numb3rs - not every episode, but on occasion. In the episode "Toxic," a private security contractor was found going through the files of a journalist the FBI was visiting. After confirming his credentials, and after the journalist declined to press charges, Sinclair let the contractor off with a warning that if they ever caught him near their investigation again, he would charge him with obstruction of justice personally. When the contractor was caught there again, Sinclair didn't charge him - he did something more drastic.
 * Another episode uses the trope: A man hires private security to find his stolen loot. The FBI is also on the case as people were kidnapped during the theft. The private security guys barge in as the FBI is about to arrest the kidnappers, which allows them to escape. Don immediately has both men arrested as accessory to the kidnappers, and warns their employer that if he sees any more of his employees following FBI agents around, he'll have him arrested under the same charges.
 * On Numb3rs, it's usually somewhat justified, as the people threatened usually are actively getting in Don's way, usually by trying to investigate themselves, not just refusing to help.
 * Subverted on The Wire. Witnesses often flatly refuse to co-operate, because as the show frequently demonstrates, the threat of being killed by an angry drug dealer is much more credible. In one episode, the cops even acknowledge that the Obstruction of Justice charge is bogus, but if you lie under oath in a Grand Jury...
 * Happens in practically every episode of Castle, which seems to contain a world full of suspects who will confess anything and everything to a firm but attractive female cop and a constantly-quipping civilian writer after enough pointed dialog. Subverted when Beckett threatens a suspect after questioning him with "don't leave town" accompanied with a threat regarding what might happen if he did leave town. Castle inquires after the fact if she can even do that, to which she responds with something to the effect of "no, but he doesn't know that."
 * The Star Trek: Enterprise episode "Judgement" could be described as what happens when the threat isn't so empty.
 * A Touch of Frost. Jack Frost, a Detective Inspector in a provincial force, regularly threatens anyone who doesn't talk to him with visits from bodies varying in importance from the local Trading Standards Authority to Interpol. Particularly unconvincing as he treats everybody not on his team so badly that he's unlikely to get co-operation from the guy in the next office along let alone people in another country.
 * Parodied in Supernatural:

Video Games

 * LA Noire: Cole Phelps and his partners make use of this trope quite some times. And it usually works out too, seeing how most people they encounter do have something to hide. Doesn't even have to be something concerning the particular investigation though; Hollywoodland was simply a Gangsterland back then.
 * In the backstory of Ghost Trick, a suspect in an espionage case was put on the spot in this way by a rookie detective during interrogation. This pushed him to, in a fit of desperation, grab a gun, flee the premises and take a hostage. The outcome of the altercation and the suspect's hopeless mindset drives the game's plot and the many deaths that occur during it. And for the record, the suspect was innocent of the allegation and genuinely knew nothing about it.

Web Original

 * Played mostly straight in the recent Whateley Universe story, Crime and Chaos. There's some evidence that the cop is purposely doing it just to get the person out of the way, and has no plans for it to actually stick. The person was, in fact, stonewalling, and the cop called a lawyer of dubious morality to handle it. This work was also an homage to Law and Order.
 * http://www.crystalhall.org/C&C.html