from JMM (above)
Now, Slap is right that "you Interview witnesses....you Interrogate Suspects." But, I would suggest that distinction arises from artificial legalisms. Once a person becomes the focus of an LE investigation, the legalisms change.
and

from Slap (above)
jmm99,for me the legalism entered into the picture when a person or persons become the focus of the investigation, that is when you have to do the Miranda thing.
Miranda entered the picture just as I was getting into the game[*]. Before that, confessions and admissions were governed by the "totality of circumstances" test. Under that test, warnings were but one factor to be considered. The FBI and Michigan State Police gave them.

The "totality of circumstances" standard (IMO) was a good one. I suppose that some disparities in results did occur - e.g., a pro-LE judge might find the "totality" OK; a pro-defense judge might find the "totality" NOK. But, the same disparities happen today, except that the rationales are legalistic and technical. The "totality of circumstances" standard looked more to the spirit of the law; Miranda and the other "brightline" tests look more to the law's letter.

It is interesting that the MCA (Military Commission Act) adopted the "totality of circumstances" standard for confessions and admissions. The DTA (Detainee Treatment Act) incorporates FM 2-22.3 as the standard, and the Exec. Order (Jan 2009) applies the manual to all US interrogations. A Wiki on the Field Manual and another on Enhanced interrogation techniques.

Regards

Mike

[*] One of my major "ghost-written" appellate briefs while still in law school was to the Michigan Supreme Court, People v Doverspike 382 Mich 1 (1969), affirming People v. Doverspike, 5 Mich App 181 (1966), using the "totality of circumstances" standard. The confession was after Escabedo and before Miranda. A lost cause - the cops did a pretty good job, even if the warnings they did give were not quite Miranda. The case is not online free; you need a WestLaw or VersusLaw account to access it.