Angry captain. Informant. Veteran cop. Medical examiner. Concerned citizen. False confessor. Overworked public defender. Reluctant witness.

priest or pastor,
psychologist who specialises in criminal profiling,
psychic,
relatives of the sleuth in snapshots of homelife,
the cast of possible culprits - usually people who know the deceased - each examined for means, motive and opportunity

Sometimes as a plot device, a crucial bit of info is discovered via a tramp or chance witness who is only accidentally discovered.

Quite often there are extras who play no role in the narrative but provide an ambience for the setting.

Innocent bystander.
Crooked cop.
Witnesses who all saw something different.
Indifferent witnesses.
Man in black hoodie.
Jealous husband.
Frustrated ADA.