Expression tags change the emotion on the face while keeping the character and composition: smile, blush, surprise, tears, wink. 119 tags are listed for V4.5.
Place them after character and outfit tags: "1girl, solo, smile, blush". Emoticon tags (:d, ;p, :3) work as written on Danbooru.
Top-scoring expression tags.
V5 follows expression tags literally and keeps the face intact when several are stacked; on V4.5 the artist's style shapes the expression more.
The expression doesn't change.
Raise the weight or add part tags such as "open mouth" or "closed eyes".