Actus reus ("guilty act") is the actual external element of a crime, the act itself that causes harm. This is to be contrasted with the other main component of a crime, mens rea ("guilty mind").
TIL that those two are the very foundation of criminal law.
Regarding that Actus Reus.... that is defined as a positive act. And from quote below: by not doing an act, by omission or failure, you seem to be not criminally liable (with some exceptions).
Omission, or failure, to act generally carries no liabilities. That means a person can only be criminally liable where they have performed a positive act.
Let’s use the classic example of person A walking past a drowning person B. Person B can be saved if person A holds their hand. Person A doesn’t hold their hand and person B drowns. Person A is not liable.
https://www.allaboutlaw.co.uk/stage/study-help/criminal-law-actus-reus-mens-rea
I find that odd.
I can understand the difficulty to provide the evidence that someone did not hold that hand "willing and knowing", but I would judge that person A as responsible if A could have done that rescue without any risk for his own life.
But perhaps this is only in UK Law, or perhaps Roman Law.