Responsibility of the witness of a crime.

I am primarily interested in finding sources from anyone published who has written on this topic. Any sort of discussion on the idea of what sort of responsibility the witness to a crime is to be held too.

I would be grateful if anyone had any links to any such journal or the like.

If you just want to talk about this topic that's cool too.

Should a witness to a murder feel/be obligated to participate in a trial to convict the murderer? Is witness protection adequate enough to force someone to participate in the trial?

If someone views a lesser evil should they be expected to report it to the authorities? i.e. theft, assault, or car crash?