Biggus Dickus is hilarious. I heard that either was improvised or the guards were seeing it for the first time during filming. At one point you can see Michael Palin almost losing it.
The Marx Brothers used to do that. There are times when you watch the films you can see the support actors looking puzzled. That's because they've tossed the script and are improvising.
Apparently that's where Harpos trademark silence came from. He had lines, completely blanked, improvised the dummy routine, and it was that much funnier he kept it from then on.