This is most likely unintentional, but there is a scene where Jess contemplates whether Leslie felt fear before she died. As a child, this seems sad, but silly as you accept the explanation by Jess' Father ( she hit her head, was knocked out and that's why she died - so she didn't have time for fear at face value. But after this troper read a couple of other books, and heard a couple of warnings about fast water, he think that actually, this explanation is improbable - by such a high stand rocks would be too deep to hit when falling just a few meters. Far more likely is that she was unable to reach the shore because of the current pull effect (current is fastest in the midlle, and anybody in water is drawn towards it) and that her limbs went numb from exhaustion and hypothermia (it was april and rain water is surely cold). So instead of a swift, almost painless death you get a long, hopeless struggle coupled with plenty of horror and despair. Good read.
The story delves a bit into the theme of escapism and how playing make believe with Leslie helped Jesse ease into his life... With her death, it leaves some to question if he would just stay holed up in Terabithia, hiding away in his imagination had his dad not come and reconciled with him.
Jess has survivor's guilt over Leslie dying while he was off with Miss Edmunds. If she hadn't invited him, he almost certainly would have gone out to play. Given the strong current aggravated by the rain, it's possible that Leslie and Jess could have been killed - especially if one fell in and the other jumped in to try and save them.
A little bit of a horrifying Fridge Brilliance that might have an explanation to why the death is changed up. In real life, Lisa Hill (Leslie's inspiration) was struck by lightning and died. Now, while this was changed to a more realistic death, keep in mind that one of the themes brought up was "what happens after a non-Christian dies?", with the overall idea that they go to hell. Lightning bolts are usually associated with God delivering judgement. Thankfully, one of the characters brings up that God's probably not going to let Leslie burn in hell, because otherwise, if the lightning bolt death was kept, some people might think that God was having his cake and eating it too.
Miss Edmunds is probably going to know that Leslie died while she was at the museum with Jess. When she finds this out, she's going to be torturing herself for wondering why she didn't invite Leslie too. Though Leslie is invited in the book too, but not in the film.
Leslie's overactive imagination makes perfect sense when one considers her parents. Not just because they're writers but the implication is that they often don't have too much time for their daughter. While they're writing, Leslie is often on her own - encouraging her to think up other worlds or ways to amuse herself.
Terabithia's creatures are full of this trope, but the Hairy Vultures have an interesting case; they're crude parodies of Gary Fulcher, whose catchphrase is "dead meat". This adds to the perfection of them being vultures. What better animal than a scavenger to have an affinity for dead meat?
Leslie's death seems sort of odd, considering her swimming skills, until you realize she probably didn't just fall in by herself, she was most likely knocked clean out when she hit the rocks.