Now, I’m from Canada, so laws are going to differ as to what’s acceptable coersion and what crosses the line (keeping the statement out of court).
Basically, the police have two ways of questioning a suspect:
–the interview, where the suspect does virtually all of the talking (ideally), with prompting and questions from the cop when he’s winding down. This is “warned,” you’ll be informed, at the least, that you don’t have to talk to the police if you don’t want. If you’re in police custody you’ll also be given an opportunity to contact a lawyer (and informed of your right to do so). The cop will most likely try to form a bond with you, and be friendly and understanding. Probably there will only be one officer present, it will be video or audio recorded. The whole point behind the interview is to get the suspect to “story up” so that holes can be poked in their story later. Any confession is a bonus, and not really expected.
–an interrogation, where the idea is to get a confession. The whole idea here is to form a bond with the suspect where he feels he can confide in you. The idea is to get him to believe emotionally he can trust you, even though he knows he can’t. The officer will do the vast majority of the talking, telling stories (I had a kid brother one time in a similar situation…), saying whatever he can think of to form that connection. The only thing the suspect is allowed to say is “I did it,” everything else gets shut down quick. IF the suspect is actually guilty, denails will steadily get weaker and eventually he may confess. The interrogater will, at all times, act supremely confident that the person is guilty, responding to any denials with a “Look, we have all the evidence. We know you did it. I just want to know why,” or whatever. The interrogater will paint two alternatives, that you’re not a bad guy and you screwed up, or you’re a souless monster. The suspect then confesses to the first so that he won’t be thought the latter.
Generally, any cop a week out of training can perform an interview (unless we’re talking about a murderer or aggravated sexual assault or something), whereas the interrogations are tricky and generally done by a cop with a little training in how to do them.
Legally, of course, they’re the same thing–all the same warnings must be given, all the same case law applies. Generally , cops in Canada cannot promise any sort of legal absolution for a confession, but can say just about anything else. I could, for example, say that people will think better of him if he confesses, or that he’ll feel better, or that his god would approve, but I couldn’t say it’ll look better to the judge or anything along those lines. The cops can absolutely lie–there’s a standard question in suspect interviews that’s pretty much always built on a lie, and the stories used in interrogations are pretty much completely fabricated.
The key legal question in Canada in relation to confessions is whether or not the suspect had an “operating mind,” thus confessing of his own free will. Rulings have upheld that things like torture remove the operating mind, but lies don’t (unless the lies enter the realm of legal consequences, of course). Thus, the bright lights, refusing to give the guy water or allow bathroom breaks, screaming in his face; it’s all dangerous waters in Canada. Probably a little of that sort of thing would be allowed, but cross the line and hours of work could get thrown out of court. Most police forces find it safer to be the nice guy, and get confessions through empathy.
Refusing to speak to the police is difficult if you’re in their custody. They can sit you down in a room and sit there and talk at you for hours if they want. You can say nothing if you want, but most people find it hard to be continuously attacked without defending their actions. You get your one opportunity to talk to a lawyer and then you’re pretty much at their mercy for however long they can legally hold onto you.
Anyway, a bit of a Canadian perspective.