In Prisoner of Azkaban, Sirius Black transforms a couple times without any special attention given to his clothes - it's either implied or pretty explicitly presented that his clothes morph with him.
When Peter Pettigrew is first revealed, he is fully clothed after having just been transformed from his animal form.
However, later in the movie, Peter returns to his animal form and leaves his clothes behind - they fall off him before he runs off.
The incident that really caught my attention though, was the train station scene with Sirius Black in Goblet of Fire (or perhaps it was a different movie/book, I'm not sure). Here, Harry must give Sirius a cloak because he has just morphed from his animal form without any clothes.
All of the above seems to represent a fair bit of inconsistency in the handling of clothes for animagi. Are there any particular rules governing when an animagus may retain their clothes throughout their transformations?
My primary experience with the Potterverse thus far has been through the movies, so it may be the case that the books handle this differently and I just don't know. Are the movies an accurate representation of the books, in this respect? Do the books offer any further explanation for this?