Some movies are known to be 'based on a true story.' These types of movies are very interesting because they actually happened. The only reason they are not considered to be a non-fiction movie is because the director or writer will add some scenes that didn't happen, or change some facts around so the movie is more appealing to the masses.
A conversation can start from movies such as Schindler's List. When coming out of a theatre, I heard a couple talk briefly about this.