It's been a while since I have done one of these. But one legality that has been kicked around lately due to the cancellation of HBO's Luck is the inclusion and safety of animals in productions. Luck was canceled after a third horse involved in the production died.
Animals, like children, are touchy subjects. If you don't want to get these guys after you, you probably need to follow the rules.
What are the rules? Well, similiar to children, animals need registered trainers or handlers on set. Every. Single. Animal. Even a roach. There are exceptions, such as shooting wildlife or documentary-style filmmaking, but for narrative filmmaking, you usually need a registered trainer. Another big rule: No animals can be killed on set. Need to step on that roach? Swat that fly? Better hire a good editor.
If you want to film your own dog, you have to prove the dog is trained enough to not cause harm to himself or others. Think about if one of your human actors gets bit by Cujo's little brother, Juco. You will have a messy lawsuit on your hands.
What about the "No animals were harmed" yadda, yadda, yadda? Guess what? You can't just use this phrase. It's trademarked. You have to get the approval of the American Humane Association, in particular, the Film and TV division.
As you can imagine, an animal on set can be an expensive expenditure. If you thought having a talking dog in that drug trip scene was a brilliant idea, you might need to sit down with your line producer, film accountant, or unit production manager and consider the cost. Otherwise, write another gag; talking dog has been done.
A complete list of laws can be found here: http://www.americanhumanefilmtv.org/guidelines/