Also an accomplished theater actor, writer and improviser, he co-Created with David Rossmer, Dan Lipton and Sarah Saltzberg the variety show "Don't Quit Your Night Job," a popular improv comedy benefit with the biggest stars on Broadway.
Co-wrote with David Rossmer, the musical "The Other Josh Cohen" which appeared Off-Broadway in 2012 and garnered 6 Drama Desk Award nominations including 4 for Mr. Rosen (Best Book of a Musical, Best Score of a Musical, Best Lyrics, and Best Leading Actor). The show then played the Paper Mill Playhouse in 2014.
Nominated for a 2015 Lucille Lortel Award for Best Supporting Actor in a Musical for playing Sam Spiegel and others in 'Pretty Filthy,' Off-Broadway, NYC.
Originated principal roles in the Broadway productions of Spamalot (2005), Aaron Sorkin's The Farnsworth Invention (2008) and the revival of Guys and Dolls (2009).