Custody: Custody can be thought of as formal arrest, or “restraint of freedom of movement of the degree associated with formal arrest.” So, if an individual is placed under arrest, then he is in custody for Miranda purposes – but this does NOT extend to investigative detentions simply because a suspect is not free to leave. However, if one is detained (not arrested) and restrained in a way associated with formal arrest (handcuffs, placed in a jail cell, etc), then he is likely in custody when it comes to Miranda.
Interrogation: This concept is also easy to determine. The easy case is where a police officer asks an individual incriminating questions. This is obviously interrogation. However, any conduct “reasonably likely to elicit an incriminating response” is also interrogation for Miranda purposes. The classic case involves two detectives conversing about the dangers of a discarded firearm within earshot of the arrestee. Although they never spoke to the defendant, their conduct was designed to cause a range of emotions in the accused which led him to disclose the location of the gun.