Aiken is making his Broadway debut in the roles of Sir Robin/Guard/Brother Maynard, which were originated on Broadway by Tony winner David Hyde Pierce. Aiken's stint is currently scheduled to continue through May 4.

Waddingham joins the Broadway show as the new Lady of the Lake in a swap with Broadway actress Marin Mazzie, who assumed that role in the London production of Spamalot. Sieber, the original (and Tony Award-nominated) Sir Galahad/The Black Knight/Prince Herbert's Father on Broadway, returns to the cast in his original roles.

The trio join a Spamalot cast that currently includes Jonathan Hadary (King Arthur), Tom Deckman (Prince Herbert), Rick Holmes (Sir Lancelot), David Hibbard (Patsy) and Brad Oscar (Sir Bedevere).

"Clay Aiken is amazing beyond that glorious voice. Turns out he is an excellent comic actor and a master of character," said director Mike Nichols in a previous release. "People will be surprised by his wide ranging talent, since the first impression is of great country charm and a singer to remember. This guy is not only a star, he is a lot more. We are lucky to get him for Spamalot."

Aiken — who was bested by Ruben Studdard during the second season of "American Idol" — has gone on to sell more than 6 million albums, co-author a best selling book ("Learning to Sing: Hearing the Music in Your Life") and play six sold-out concert tours. He was a focus of the short-lived Off-Broadway musical Idol: The Musical and has appeared on "Ed," "Saturday Night Live" and "Scrubs." Monty Python's Spamalot, which celebrated its 1,000th performance Aug. 9, 2007, opened at Broadway's Shubert Theatre on March 17, 2005, following previews from Feb. 14.

Mike Nichols ("The Graduate," "Angels in America") directed and Casey Nicholaw (The Drowsy Chaperone) choreographed the show that was inspired by the 1975 feature film comedy "Monty Python and the Holy Grail."

The musical tells the tale of King Arthur and his Knights of the Round Table on their quest from God for the Holy Grail - with a short stop in the dazzling Spam-filled land of Camelot.

Spamalot earned the 2005 Tony Award for Best Musical and took home honors for direction (Mike Nichols) and one of its stars (Sara Ramirez). The musical also enjoys runs in London, Las Vegas and a national tour. An Australian production opened in Melbourne in November.

For information on all Spamalot incarnations, visit montypythonsspamalot.com.