Actor Shuler Hensley, who plays the Critic in “The Guardsman” in its run at the Eisenhower Theater for the next month, is one of those rare actors who is never idle. He arrives at the Kennedy Center fresh from a successful Off-Broadway run in “The Whale,” for which he won the Lucille Lortel Award for Lead Actor and nominations from the Outer Critics Circle, the Drama League and Drama Desk for Outstanding Actor in a Play. New York Magazine’s Year in Culture 2012 honored him with Stage Performance of the Year.