There are two basic rules to consider: facts can't be copyrighted and most resources we want to use for the OFDb is copyright protected. While this might sound like a contradiction, this is the reality we are confronted with.
I can freely use single facts from a website and add them to the filming location database. However, a collection of facts is copyrighted and can't be added.
Let's look at 2 real life examples:
You want to add the movie Ich seh Ich seh to the OFDb. The first resource, of course, is the movie itself and the credits. In addition, you see that there is an official website where you can find information like production country, aspect ratio, distributor, and so on.
You may use this information - both from the movie credits, and from the website -, as it is considered a single fact: information about a single movie, taken from its credits and official website.
Now you want to add Adú to the database, but can't get all information you'd like to add from the movie credits. You know the movie is distributed by Netflix, where you find some the information you want.
However, this is considered a collection of facts and its data can't be added to the OFDb: Netflix's website contains thousands of titles.
As a rule of thumb, single facts are only ok to use, if you take them from the credits or the movie's or TV show's official website (if that website is only about his specific movie/TV show). The website of a streaming provider like HBO or Netflix, for example, is copyright protected, because it is not considered a single fact, but a part of the streaming site as a whole.
If in doubt, please don't add the data. The OFDb is meant to be truly free data without the reputation of dodgy practises. Eventually, this will do more harm than good.
There are free licenses that might be used without asking for consent. The OpenStreetMap wiki has a list, which licenses are compatible with the ODbL. Since they use the same license as the OFDb, it is also applicable for us.
Be aware: some licenses that are compatible might need attribution. Only add data from those sources if you can comply with its attribution requirements.