Douglas Adams wrote the role of Arthur Dent with Simon Jones in mind. They had worked together in 1976 on a show called "Out of the Trees."
I'd much rather be happy than right, any day.
Arthur Dent: And are you?
Slartibartfast: No. That's where it all breaks down, of course.
The person operating Zaphod's third arm can be seen on multiple occasions.
After the credits for episode 3, the voice of the book returns briefly to resolve which one of the characters sustained a bruised arm -- a question that had been raised earlier in the episode, but left unanswered because it was unimportant.
The first episode, when it was shown to a live audience for the purpose of recording a laugh-track (see above) included an 8:30 comedic video introduction by series narrator Peter Jones. This sequence was never broadcast on TV, but has been included as an extra on the Warner Brothers DVD release.