Saw a programme a year or so after Eric's death, where Eddie Braben was talking about what it was like working with the pair. He talked about the famous piano sketch, featuring Andre (Mr Preview) Previn, and said that Eric in particular spent HOURS just walking backwards and forwards on the set, moving the piano and stool backwards and forwards, tripping on different steps, until he was satisfied that he had made the whole thing as funny as it could be. Braben said that this was fairly common.
And i remember seeing an interview with Andrew Preview
who said at the time he was too busy to rehearse the sketch as much as M&W wanted and that stressed Eric out. The genius of the two of them was to make everything look so ad libbed when in actual fact the show was rehearsed to within an inch of its life.