Meaning:
“入” means “to enter,” “戏” means “drama,” “太” means “too,” and “深” means “deep.” Literally translated as “getting too deeply involved in a drama,” this term can refer to scenarios where the actors or audiences get so personally emotionally involved in a film or TV show that they blur the line between the fictional plots and reality. Sometimes, it’s used in a more general sense when people become too devoted to or too serious about something that isn’t worthwhile.
Example:
A: 小马怎么最近天天垂头丧气的？
Xiǎomǎ zěnme zuìjìn tiāntiān chuítóu sàngqì de？
Xiao Ma recently seems to be in low spirits. Any idea why?
B: 因为丽丽不理他了。
Yīnwèi lìli bù lǐ tā le。
That’s because Li Li broke up with him.
A: 只怪他入戏太深，丽丽本来就是把他当备胎，现在又和前男友复合了。
Zhī guài tā rùxì tàishēn，lìli běnlái jiùshì bǎ tā dāng bèitāi，xiànzài yòu hé qiánnányǒu fùhé le。
He’s been too serious about this relationship. Li Li only agreed to see him because she’s on the rebound, and now she’s getting back with her ex.