When do police actually have to read Miranda?

Two conditions, and both have to be present.

Custody means a restraint on freedom of movement of the degree associated with formal arrest. It's assessed objectively — whether a reasonable person in that position would have felt free to leave — rather than by what the officer intended or what label was used.

Interrogation means express questioning, or words and actions the officer should know are reasonably likely to elicit an incriminating response. That second part matters: a conversation designed to prompt a statement can be interrogation even without a direct question.

So no warnings are required when an officer asks questions during a non-custodial encounter, or when an arrestee volunteers a statement nobody solicited. Spontaneous statements are generally admissible.

There's also a narrow public safety exception permitting limited unwarned questioning where there's an immediate threat, such as an unsecured weapon.