He was 33.
Hallett had called 911 Sunday evening complaining of shortness of breath, said Ed Winter, a coroner's spokesman.
The actor was rushed to the hospital, where he was pronounced dead at 8:35 p.m.
An autopsy is pending.
Hallett was born in 1975 in the village of Osterville, Mass., part of the town of Barnstable, on Cape Cod. He attended Assumption College in Worcester and studied business.