I think you can make the reader believe almost anything in fiction if you write it right. But fiction loses its interest to me if it becomes unbelievable. If someone does something no one would do, or drastically out of character without explaination or reason for doing such, then the character loses the humaness and becomes something unknown. Characters should never do something because it is "in the script"* The people are living the story, not playing a part. People don't decide what their actions will be by what makes a story interesting. "Not in the script": In the military during a military exercise, things happened that could not be explained, "enemy" were immune to capture or being killed even when in plain sight. So I explained it to our side, "Don't argue, it's not in the script."
