Shiver will play Dean O'Banion - based on a real-life florist-gangster in Chicago who was also Al Capone's adversary - while Steedle will play a Broadway actress named Billy Kent, reports Deadline.
Shiver, whose credits include roles in Fright Night, The Book of Eli and Brothers, can be seen next in A&E's new drama Longmire.
Boardwalk Empire is Steedle's second television role. She previously appeared in ABC's Body of Proof.
In January, Boardwalk Empire's star Steve Buscemi was honoured at the Screen Actors Guild Awards for 'Outstanding Performance By A Male Actor In A Drama Series'.
Boardwalk Empire also won the award for 'Outstanding Performance By An Ensemble In A Drama Series'.
Watch the trailer for Boardwalk Empire's second season below: