From the DVD commentary, episode 1:
Neil: The wonderful thing about Good Omens all the way through was the amount of goodwill. There was love for the book, and people came in because they wanted to do it. You know, Danny [Daniel Mays - Arthur Young] is amazing. There's Nick Offerman [Thaddeus Dowling] who offered to fly himself to South Africa to be in it, and we turned him down and we flew him, but he came to South Africa for two days of shooting in a tiny part, because he loved Good Omens, because he wanted to do it.
Douglas: And our first AD [assistant director] said to him in South Africa, 'You've come a long way for only a few lines.', and he said, 'I would have come twice as far for half the lines.'