This tells us more about your stereotypes of criminals than anything else. If you made a story up out of whole cloth to impress someone it would be unlikely to fit with any other evidence in the case. Back in reality, if you read cases where such tactics are employed, they usually involve pairing someone up with a cellmate who is in for some relatively minor crime and gain's the suspect's confidence by talking freely about their own situation. Admissions to cellmates usually come after days or weeks, once the suspect has become comfortable, at which point the suspect may brag about their criminal activities - in much the same way as everyone else likes to share 'war stories' about their line of work from time to time.
A threatening or scary cellmate that glared aggressively at a suspect would be a gift to defense lawyer, since it would make it a lot easier to argue that the confession had been coerced and should therefore be suppressed. Criminology texts emphasize this point; the object is to elicit a confession that will stand up in court. In the vast majority of them the informant presents a friendly demeanor towards the suspect, and furthermore is instructed not to prompt or ask questions but simply to pay close attention to the suspect's conversation and report anything interesting to investigators - again, to avoid complaints that a confession was coerced or that a defendant was put in a position where they felt obliged to provide information without benefit of counsel. Indeed, law enforcement often uses undercover officers posing as prisoners, and courts don't consider this a violation of Miranda rights as long as statements made by a suspect are voluntary.
That makes much more sense than the big tough guy that I mentioned. But there are still many reasons why an innocent person thrown in jail could make up an story. I guess that it would be OK to use that confession if it leads the investigator to new evidence, but using it in court as an evidence in itself seems very wrong to me.
My knowledge about criminology or criminal justice is very limited, and it's probably polluted by all the cop movies and TV shows. So any input from somebody more knowledgeable on the subject is more than welcome.
Your instinct about using it in court as evidence is basically good - the 'hearsay rule' forbids testimony of the form 'so-and-so told me that the defendant was guilty,' with a few very narrow exceptions like the last words of a dying person that accuse someone of a crime. Because of teh 6th amendment, informants and undercover officers have to testify in court, where the defense can question them and challenge the veracity of their statements.
Of course this is not perfect. There have been cases where law enforcement or prosecutors told the informant what to say in an overzealous effort to gain a conviction, and sometimes innocent people have languished in jail for years on the basis of such false testimony. That shouldn't be allowed to happen, but when it does it doesn't necessarily mean the entire judicial process was corrupted. For example, a trial may have been properly conducted but if the defense lawyer was lazy or incompetent and failed to challenge the prosecution evidence the judge and jury may not have had any way of knowing about the flaws int he prosecution's case.
One important thing to bear in mind about American courts is that trial courts do not have an investigatory function to discover all the information about a case, but rather a decisive function to select between competing arguments from plaintiffs/prosecutors and civil/criminal defendants. So even if the defense lawyer is an obvious imcompetent the court inn't supposed to pre-emptively assist the defendant - the defendant has to ask for a new lawyer or bring up the issue at appeal.
In many other countries they use a different sort of system called civil law, where the judge is basically the lead investigator in the case. That can prevent some kinds of miscarriages of justice that occur in our system but as you might expect it opens to the door all sorts of other kinds.
This tells us more about your stereotypes of criminals than anything else. If you made a story up out of whole cloth to impress someone it would be unlikely to fit with any other evidence in the case. Back in reality, if you read cases where such tactics are employed, they usually involve pairing someone up with a cellmate who is in for some relatively minor crime and gain's the suspect's confidence by talking freely about their own situation. Admissions to cellmates usually come after days or weeks, once the suspect has become comfortable, at which point the suspect may brag about their criminal activities - in much the same way as everyone else likes to share 'war stories' about their line of work from time to time.
A threatening or scary cellmate that glared aggressively at a suspect would be a gift to defense lawyer, since it would make it a lot easier to argue that the confession had been coerced and should therefore be suppressed. Criminology texts emphasize this point; the object is to elicit a confession that will stand up in court. In the vast majority of them the informant presents a friendly demeanor towards the suspect, and furthermore is instructed not to prompt or ask questions but simply to pay close attention to the suspect's conversation and report anything interesting to investigators - again, to avoid complaints that a confession was coerced or that a defendant was put in a position where they felt obliged to provide information without benefit of counsel. Indeed, law enforcement often uses undercover officers posing as prisoners, and courts don't consider this a violation of Miranda rights as long as statements made by a suspect are voluntary.