The only thing I wanted to do when I was a young naive kid was to become a New York stage actor.

Platt won the New York Film Critics Online Award for best supporting actor for his role in Casanova. The play was set in Dublin and Platt's role was the tortured protagonist John. He also played a lard merchant named Papprizzio in Lasse Hallström's Casanova who competes with Casanova (Heath Ledger) for marriage to Francesca (Sienna Miller).

He is starring in the Showtime original series The Big C with Laura Linney.