in the act of 固定搭配用法

It was a picture of a boa constrictor in the act ofswallowing an animal.
字面翻译：
这是一幅画，画里有条正在吞食野兽的大蟒蛇
优化翻译：
画里有条大蟒蛇正要吞食野兽。
-- 摘自《小王子》

in the act of doing sth

in the act of 可以看成是一个固定的表达，就是正在做某事的意思或表示正在做某事的过程中.

Ken was in the act of paying his bill when Neil came up behind him.
当尼尔走到他身后时，肯正在付账。

The student was caught in the act of cheating.
该学生作弊时被抓。

I caught him in the act of reading my private letters.
他正在读我的私人信件时，被我当场抓住了。