I'm confused about this sequence - midway through the movie Harry Potter and the Prisoner of Azkaban, Harry is seen with Sirius Black at the lake with a bunch of dementors. He tries casting Expecto Patronum and fails, and then the stag (or doe, I kind of forget at the moment) Patronus comes out and saves him.

Harry claims to have seen his father.

Later on in the movie Harry and Hermione are back at the lake on the opposite side, and this time Harry is the one who casts Expecto Patronum and scares all the dementors away.

Where is the stag in this scene?

I thought Snape was the one doing this, not Harry Potter? Are they both doing it at the same time?

I'm confused about this sequence – midway through the movie Harry Potter and the Prisoner of Azkaban, Harry is seen with Sirius Black at the lake with a bunch of dementors. He tries casting Expecto Patronum and fails, and then the stag (or doe, I kind of forget at the moment) Patronus comes out and saves him.

Harry claims to have seen his father.

Later on in the movie Harry and Hermione are back at the lake on the opposite side, and this time Harry is the one who casts Expecto Patronum and scares all the dementors away.

Where is the stag in this scene?

I thought Snape was the one doing this, not Harry Potter? Are they both doing it at the same time?