機嫌 and 表情: mood, temper, and facial expression
Compare a baby's mood and a manager's temper with a brightened face and an actor's practiced sad expression, keeping expression distinct from actual feeling.
Mood or temper with 機嫌
機嫌 (きげん) refers to mood or temper. “昼寝の後、赤ちゃんの機嫌がよくなった。” says that the baby was in a better mood after a nap.
In “部長の機嫌を気にせず、必要な意見を伝えた。”, the speaker voiced the necessary opinion without worrying about the manager's mood. Preserve 気にせず rather than saying that concern about the manager prevented speaking.
A brightened face and a practiced face
表情 (ひょうじょう) is facial expression. “知らせを聞くと、彼女の表情が明るくなった。” says that her expression brightened when she heard the news.
“俳優は鏡の前で悲しい表情を練習した。” describes an actor practicing a sad expression in front of a mirror. The practiced object is a facial expression; actual sadness is not stated.
Do not automatically equate a face with a feeling
The baby whose mood improved and the person whose expression brightened illustrate the targets of 機嫌 and 表情. A face can relate to a feeling, but the sentences directly describe different things.
練習した is especially useful in the actor example: it separates a practiced expression from an actual emotion. Do not define 機嫌 as only outward behavior or 表情 as always unrelated to feeling.
Does practicing a sad expression establish that the actor actually felt sad?
Show the answer and why
No. The example describes practicing a facial expression in front of a mirror. It does not establish the actor's actual emotional state.