Ron Howard's role as the narrator was accidental. He was merely filling in on the pilot, but his voice just "worked."
Neither David Cross's nor Jeffrey Tambor's characters were ever intended to be regular characters. It was not until the actors tested well that the writers ended up putting them regularly in the series.
Michael Cera, who is Canadian, had problems with his work visa, and was almost unable to complete work on the pilot episode. Cera had to go to Tijuana, Mexico in order to obtain another visa. Michael Angarano was on standby to replace Cera, in case he was deported.
Nearly every episode begins with "Michael" or "Michael Bluth" as the opening word(s).