Sieber's most visible role in the Monty Python-inspired musical is that of Sir Galahad, the flaxen-haired knight who sings "The Song That Goes Like This." But the actor wants to make it clear that there's more to his performance than a great wig. "A lot of people don't know that I play two other parts in that show: The Black Knight and Prince Herbert's father. They say 'Oh, who's that guy,' and 'Where's Galahad? Where's he been?'" Of the three roles, he has a particular affection for Herbert's gruff pap. "I love the father. You have to kind of listen, because you can't recognize me. I'm in this beard and moustache and the 45-pound fur costume. I look huge."
Sieber was the last major cast member to be added to the Spamalot company, joining the show after Douglas Sills made an exit. (A good friend of Sills, he joked "I'd like to thank Doug for dropping out of the show.) Offering a comic version of how the part came his way, he said, "They called up and asked if I'd like to do the show. I said, 'Sure. Why not? Work with Mike Nichols? Maybe. Eric Idle? I don't know, but I'll do it. But I won't enjoy it! As a favor, I'll do it.'"