Hallet was originally the personal assistant of Joss Whedon's wife, Kai Cole. However, after Whedon saw Hallett singing in a Universal City Blues revue, he conceived the character of The Host, the anagogic demon who reads people when they open up through singing karaoke. Hallett was invited to try out and got the part, his first job ever as an actor. Andy has enjoyed working on the series Angel and has been known to help his fellow cast mates out with their own karaoke singing.
Hallett got his agent after he appeared on Angel. She spotted him singing and acting in the second season premier episode "Judgment". The morning after the show was broadcast she called up the series casting agent and asked about him. He signed with her shortly after.
A month after the Angel series finale, Hallett suffered a dental infection that spread to his heart and caused a five-day hospitalization for cardiomyopathy. The infection weakened his heart's muscle and valves, causing severe fatigue and three additional hospitalizations over the next five years. He died of congestive heart failure on March 29, 2009 at Cedars-Sinai Hospital in Los Angeles, with his father Dave Hallett by his side.
- "Dear Boy"
- "Dead End"