Mandarin: when to use 面 (mian) or 臉 (lian)?
In modern Chinese, when it comes to the meaning of &quot;face, they basically are the same. However, in classic Chinese, they mean differently. Knowing the origin of them may help you understand how they are used in words.

面 meant &quot;face&quot;; however, 脸 is used to refer to face now.
脸 meant &quot;cheek&quot;; however, 脸颊 is used to refer to refer to cheek now.

Below is an abridge of a Ci poem written by 晏殊 in Song dynasty

原是今朝斗草赢,笑从双脸生。

Loose translation:

It's the case that (she) won the game of 斗草 this morning, (so) smiles come out of her two cheeks.

As you can see, a man can not have two &quot;faces&quot;.
Since southern dialects remain more classic Chinese elements, you can find 面
more when it comes to face. As far as I know, in shanghai dialect, 面 is also used that way.
So a rough guideline is:
in literary and archaic words (for example, idioms), 面 is used more; while in colloquial words, 脸 is used more.
For example, there is a word 面目 (meaing: appearance,looking). However, you would say 脸和眼睛 when you refer to &quot;face and eye&quot;. Regarding your example of &quot;面对面&quot;, you can actually say &quot;脸对脸&quot;. However, they are different.
you can say：

我想和他面对面地谈一谈。I wan to talk with him face to face.
他们俩脸对脸地站在那里。They two stand over there face to face.

In case 1,&quot;面对面&quot; actually means &quot;physically close&quot; (e.g., not on the phone). Your face is not neccessarily opposite his face. While in case 2, &quot;脸对脸&quot; means &quot;facing each other&quot;, so their faces are opposite. If you use &quot;脸对脸&quot; in case 1, it sounds very strange. If you use &quot;面对面&quot; in case 2, it sounds too formal.