The murderer drowns in the middle of the lake, but when his/her body is dragged to shore, it is completely dry. Also, his/her eyes can be seen moving slightly.
When Lady Felicia sneaks into the police inspector's office to peek at the coroner's report, she discovers that the victim died of atropene poisoning instead of strangulation. But if you stop the video, the report is regarding marks on the victim's neck, indicating strangulation.
Father Brown says to Raleigh, "Judge not and you shall not be judged." He is misquoting Matthew chapter 7 verse 1 - "Judge not lest ye be judged" (or sometimes "Judge not that ye be not judged"). Either way, it conveys a slightly different sentiment to what Father Brown said.