I didn't recall seeing Sadler in any roles prior to this. Having an unknown in the role helped enrich the mystery of the character for me. That, plus he did a great job in that role.
He'd been floating around TV as secondary characters for a while.
I remember him being Dan's idiot buddy Dwight from the first couple of seasons of Roseanne.
He was also in Die Hard 2 and The Shawshank Redemption.