As far as I am concerned, the 850 (in all reality) played only a minor part in most films. Fortunately, there is such a thing as the Internet Movie Car Data Base where you can report cars from movies.
I already wrote an item on that earlier on and there are enough Fiats in the list. Me, I'm only interested in the Fiat 850, in all variants.

In Dutch movie "Turks Fruit", for example, an 850 Special was used as an extra:

Een Fiat 850 Special in de fil Turks Fruit

The database can be found here. You'll have to browse through it yourself but to make things easier I have put some links in the table below.

Berlina 35 pages
Berlina Vignale 4 photo's
Familiare 3 pages