In Monty Python's Life of Brian Spike appears in just one scene and says very little. But the scene is side-splittingly funny, arguably the funniest scene in the movie.
Spike plays a prophet ignored by the masses. (He is in the middle of the shot in the brown oufit). Watch his reactions to all that is going on around him. Just hilarious.
His appearance was completely unplanned. By chance, Milligan was visiting World War Two battlefields in Tunisia, where Life of Brian was being made, and was invited to join the scene. After shooting the scene Milligan disappeared off set and could not be found for later scenes.
Here is another Spike Milligan classic- The Irish Olympics