通常来说in the face是指来脸上的外来异物.on the face是指脸上本来就有的.
例如：①There is some water in your face because of the tiny rain.（你的脸被细雨打湿了）
②As usual,there are two eyes and a nose of course a mouth on human being's face.(通常来说,人类的脸上都长了两个眼睛,一个鼻子,还有一个嘴巴）