In 1945, Rome was in a state of devastation after years of war. Mussolini was gone, as were the Nazis, but it was only a matter of months since the Allies had forced the Nazis to leave. Rossellini had the locale for his documentary-style drama about occupied Rome. What he didn't have was money. So Open City has a slapdash feel by necessity. Rossellini created an early neorealist classic in part because he had no other choice.
But it is a mistake to think of Open City as lacking artifice. I found the score, by Rossellini's brother Renzo, to be obtrusive and at times melodramatic, especially given the realist feeling of the film:
Anna Magnani is an interesting choice as one of the few professional actors in the film. She has the look of an ordinary Italian, but her acting style can be florid. In Open City she can be quiet and then erupt into volcanic emotion. She is at times the best thing about Open City, but you are usually aware that she is acting, quite noticeable compared to some of the other actors.
Here is an excellent video essay on the film by Bruce Isaacs:
#124 on the They Shoot Pictures, Don't They list of the top 1000 films of all time.