The police captain Cao Gang was play by the actor who was kidnapped in 2004 in the real event which this movie is based on.
The guy wearing headphone during the raid on massage parlor was the director himself, Shen Ding.
The actor Ye Liu took a call from his son at the end of the movie. The subtitle only showed he said "son". In Chinese he actually said "NouYi" which is the name of his real son.
This film has 100% rating based on 10 critic reviews on Rotten Tomatoes.