The show follows a young Carr, played by Oliver Savell, as he navigates secondary school during the mid to late-1980s.
He previously told the BBC the sitcom, which he co-wrote, was a “love letter” to his hometown.
Sullivan told BBC radio presenter Babs Michel that during the first series, Carr would give her advice saying: “Oh no, my mum wouldn’t say that.”
He also helped when the cast ad-libbed, she said.
But Sullivan revealed by the third series they had a better grasp of the characters, and the comic “was very much like ‘you’ve got it, you go for it’.”
