Originally, Klaus was going to be French, and named François. When Dee Bradley Baker auditioned, he told the producers that his French wasn't very good, and that he would read the lines in German instead, they liked his performance so much, they retooled him so he was German.
Steve's original design was much geekier and gawky, and he was voiced by Ricky Blitt. Halfway through production, his voice actor changed to Scott Grimes, and his design was made to be more attractive, so he would be less comparable to Neil Goldman, from Family Guy (1999). This design is a caricature of Series co-Creator, Mike Barker.
In season one, episode four, "Francine's Flashback", we find out that Francine was twenty in 1985. We can draw from this, that Francine was born in 1965, and that on the show, she is around forty-years-old. This was confirmed in season one, episode twenty-three, "Tears of a Clooney", in which it is Francine's thirty-ninth birthday.
Sir Patrick Stewart (Avery) also appeared on Family Guy (1999) season seven, episode eleven, "Not All Dogs Go to Heaven", not as Avery, but as himself. The same thing happened on this show, season eight, episode ten, "Blood Crieth Unto Heaven", in which he played Avery and himself.
Hayley (Rachael MacFarlane) says in season one, episode three, "Stan Knows Best", that she is eighteen-years-old. Based on this information, we can assume that she was born in either 1986 or 1987 (depending on her month of birth).
From season five, episode twelve, "May the Best Stan Win", it was revealed at the end of the episode exactly when Stan and Francine were born. Stan was born in 1967, and Francine in 1971. We see this information written on their gravestones when they died and are buried next to each other. Today, 2017, Stan is fifty-years-old and Francine is forty-six-years-old. Both will die in 2067, Stan will be one hundred-years-old, and Francine will be ninety-six-years-old.
The uniformed officer, in the Underdog Productions logo at the end, says something different in every episode. In the season seven finale, he was given a speaking role as Steve's imaginary friend "Pedro".
In season three, episode two, "Meter Made", Francine asks her sister Gwen, "How are you enjoying being three years younger than me?" Later, in season ten, episode eight, "Now And Gwen", Gwen is a senior in high school, while Francine is a freshman, making her roughly three years older.
The final frame of the introduction was changed after the first season. At the end of the credits in the first season, all that could be seen was a big "American Dad!" logo in front of the American Flag. In all subsequent seasons, this logo was shrunk to fit the names to the series creators on the same screen.
In the season two opener, "Camp Refoogee", Stan flies to Africa and turns a refugee camp into a summer camp. However, in season seven, episode seventeen, "Ricky Spanish", a sponsored child from Africa, named Tungee, comes to live with the Smiths. Stan asks, "Does this mean Africa's real?"