The Elements of a Crime

When a person is accused of a criminal act, government must prove, beyond a reasonable doubt, that:  a crime did occur and that it is the accused who committed the act (actus reus) and that the offender intended to and purposefully committed the act (guilty mind or mens rea). 

On the other hand, there are some instances when a person can be accused of a crime if they fail to commit an act.  Good Samaritan-type legislation, for example, holds a citizen accountable if they do not stop and lend assistance when they see that someone has been injured (such as in an automobile accident).  Another example is finding that your roommate has attempted suicide by taking a bottle of sleeping pills and is unconscious on the living room sofa.  Under some state laws, you could be charged with failure to lend assistance if you did not call 911 and get her medical assistance. 

In sum, an act is not always necessary for a person to be charged with a crime, but the majority of laws are built upon the notion that an act occurred and that the offender intended to commit it.