He has served as director of the live improvised soap opera Die-Nasty, and has been a core member of the troupe since its founding in 1991.
In 2005, Andersen exported the Die-Nasty company's annual Soap-A-Thon to England, working with legendary British theatre artist Ken Campbell to produce a 36-hour-long soap opera in London.
In January 2008, Andersen returned to London (along with several members of the Die-Nasty troupe) to direct the 50-hour Improvathon at the People Show Studios.
He was joined by Canadian improvisors Mark Meer, Kory Mathewson, Matt Alden, Kurt Smeaton, Davina Stewart, Jamie Knifefight Cavanagh, and Belinda Cornish, as well as musical director Jan Randall.
Andersen is the recipient of a Dora Mavor Moore Award for co-writing the revue Not Based on Anything by Stephen King with Second City.