MKE Prosecutor wrote: Miranda only needs to be read to you once you are "in custody." If you are not in custody you don't get the warning, but can still assert the 5th.
This is partially true, but incomplete and a little misleading. Miranda is triggered prior to "custodial interrogation" i.e. when police want to question someone in custody. Some states (Calli - see W&I code sec 625) have laws that require Miranda warnings upon taking custody of juveniles regardless of whether they will be questioned, but assuming an adult is arrested, Miranda is only required if the suspect is in custody and questioned.
No - you can't assert the 5th if you are not in custody because the 5th amendment privilege against self-incrimination and the court contrived Miranda rule that flows from it only applies to those that are in custody (or testifying in court). You can, however, tell the police to take a flying leap, call them pork-like products and otherwise remain silent in or out of custody. Although I discourage insulting the police, especially if you are in custody, remaining silent is almost always your best option.
For the OP - suspects do not "always" talk. A significant # do for the reasons stated in this thread and many, many more. Most of the time, they have a story to tell that they think mitigates their involvement in the crime. Other times, they simply can't help themselves. And some just don't give a F. You would also be surprised at how a good Detective can get someone to trust and talk to him/her regardless of Miranda. It's all a game in the end and if you find yourself on the wrong end of a police interview table, the best thing to do is ask for a lawyer and shut the F up.