Continuing our discussion of the initial contact non-custodial (i.e., not under arrest) interrogation, let's look at some of the tactics you might be facing. There is a lot of truth to the good cop/bad cop maxim. Sometimes this can be the same person. Often a person will be presented a situation in which the detective or agent will present themself as the only hope for you. This is false and is designed to get you to believe that the illusory relationship exists so you will make the incriminating statements sought by this interrogator. Also note that I use the term interrogation and interrogator not the law enforcement preferred "interview". YOU are not being interviewed you are being interrogated. This is not a job interview but law enforcement attempting to get enough evidence against you to charge you with a crime. An interrogator has no authority to promise you any benefit from talking to them. More common is the vague promise, the "I can help" without saying what that help actually is. It is a crime for you to lie to law enforcement, it is called "obstruction". Remember it is not a crime for them to lie to you. If they lie to you and you make ill advised statements then you will suffer those ill effects. The bad cop is less likely today than the good cop, threats and predictions of dire consequences for not talking to them have largely disappeared in favor of the "be your friend" approach.
Finally, don't be misled by the "your just a witness" ploy. I have represented several individuals who began as "just witnesses" and ended up defending against charges when the target case fell apart. I recently represented a person in this "witness or defendant posture" being pressured by the FBI. He retained me and I provided him the tools he needed and the representation to make the FBI go away. He was never charged. I provide clients a form titled Assertion of Rights which, among other things, directs law enforcement to not talk to my client but to call me. I then tell them we are not talking to them in most instances.