I know this is not something most brickfilmers do,  but I used to not be a brickfilmers and tried this wih the blokes in my neighbourhood... And it is a great idea! Do not listen to those who tell you not to do this because despite its efficiency for brickfilms it is great fun and quite the amazing finished product.
That being said, I have a few suggestions and tips to make your set as realistic as possible. I never had this problem because my city was very very small, but I saw this reflected in the cities of my neighbors. Do not rely only on official lego sets. If you do, your city will end up looking really shady and sketchy, like the bad part of town that nobody wants to live in. I would recommend deciding on what sort of town you are making. Is it a busy urban downtown area with skyscrapers and public parks, or a quiet suburban neighbourhood. (If it is the latter then I would suggest buying a lot of the lego Beach House, as they are very good designs. Do not forget to add all the little details like street lamps, trash cans, trees, benches, and the little lines that they paint on the roads. Also if you are designing it minifig scale you gotta make cars and everything 6 studs wide because that's the standard. Finally, do not be afraid of imperfections. Every city has them, except Singapore. But it will make it a lot more realistic! Add like construction sites or litter or graffiti--it will add more character to the city. Also make sure your camera can fit in places to film.
So yeah, that is the advice I can give to you. All I can say now is good luck!