Mr. Janson was 20 minutes into the show, a production of the Remy Bumppo Theatre Company at the Victory Gardens Greenhouse Theater, when he put his head in his hands. When asked if he was OK, he replied no. He was rushed to Lincoln Park Hospital and died soon afterward, with members of his family surrounding him. He had suffered a heart attack. Mr. Janson played a former U.S. President in the Vidal drama. Ironically, his character dies midway through the play.

"There is a certain poetic irony to his death," Christopher Janson told the AP. "He died doing what he loved, which was being on the stage and in a play he was so proud of."

Genial and open-faced, with a gentle, calming voice, Mr. Janson acted in dozens of plays. In recent years, he performed at smaller companies like The Artistic Home and Chicago Dramatists.

He also performed at the Apple Tree Theatre, Red Hen Theatre, Element Theatre and New City Theatre. Motion pictures include "The Blues Brothers," "While You Were Sleeping," "The Killing Floor," "My Best Friend's Wedding," "Blink," "Road to Perdition," "The Kill Reflex" and "Ocean's Twelve."

He is survived by his wife, Peggy; a son, Christopher; two daughters, Maureen Heintz and Colleen Wares; and four grandchildren.