I was watching Raise the Red Lantern (which is an excellent movie by the way) and saw this phrase come up on the subtitles, but wasn't able to catch what the actual Mandarin behind it was. Does anyone know what it translates to?
I didn't see this film, so I am not sure if this is the original one used in the film.
From your description, I guess the idiom is "人面兽心", which literally means "a human's face and a beast's heart", actually is used to describe a man who looks kind, but really is vicious, evil or wicked.
By the way, I think you can show us the time point or the scene when this line appears so we can easily catch it for you.
Update: The original word in the film is "菩萨脸,蝎子心". I don't think it's an idiom. I'll treat it as an analogy, a figure of speech. Actually, "菩萨" isn't Buddha, but Bodhisattva, an image in Buddhism. The subtitle translated the meaning well and I think it's easy for you to understand.
If you insist to have it translated into the idiom, I think
佛口蛇心is the most suitable. It is the closest in meaning, and really is an idiom.
面如桃李 心如蛇蝎。I think this idiom should be appropriate, although buddha is 佛陀 in Chinese, but 桃李 means the flowers of peach and plum.
The face of a Buddha, the heart of a scorpion means someone who appears to be kind and gentle but who in an instant will deliver a mortal wound by word or by action after another trusted them. In the film it is clearly to see why the warning comes to the young bride.
I had a Chinese friend in Hawaii who used to tell me "beware of those who smile at you to gain your trust, before you give them your trust make sure their hands are in front of you" which she explained it was her Grandmother's way of saying that before you trust someone you need to know them well to make sure they are worthy of your friendship. The hands in front of you meant that they would not be able to hide a dagger to later stabbed you with.
By the way, I saw that movie many years ago when it came in and promptly bought it for my collection. A real treasure of Chinese films.
笑面虎 may be what you are looking for
EDIT 佛面蛇心 may be another term for what you are looking for