The man who sold Alfons a Currywurst at the funfair and introduced himself as Alfons is Helmut Rossmann. He was playing Alfons Zitterbacke in Alfons Zitterbacke (1966), the first movie adaptation of the kids book series.
Alfons' April Fools prank about him being drunk and calling the teacher "woodhead" is a combination of two separate chapters of the first book. In the chapter "What an April Fools prank gave me" he was telling on how he called his teacher the name and was threatened to be kicked out of school. In the chapter "How I was a fake drunkard" he was telling on how he and his friend Bruno (Benni in the movie) were pretending to be drunk in a restaurant due to booze.
Alfons' father's full name in the movie Alfons Paul Bruno Zitterbacke. It is a combination of various characters from the book series. Bruno was Alfons's friend. Paul was his father, according to the first book. And in the third book the character Alfons was the son of the Alfons from the first two books.
In the movie Alfons' father thinks he is 12 while he is actually 11. In the book he thought Alfons is 11 while he was actually 10.