The phrase ‘going to the mattresses’ has nothing to do with the horse head scene. I think it is Clemenza who says it when the family is talking about retaliating against Tattaglia and Sollozzo in reference to their all their hitmen hiding out together in a bare bones apartment only filled with a bunch of mattresses where they are awaiting further instructions. The phrase means ‘to prepare for battle / war’

THE GODFATHER IS AN ABSOLUTE MASTERPIECE

The horse head scene not only showed that Corleone could take something valuable from him, but if they could slide a horse head into his bed without waking him up, how easily could they just kill him? He realized at that point that he'd messed up.
Also, in the book, when Sonny was killed, Tom called Carlo and told him he had to be a model husband because they suspected him in Sonny's death.

For an old Italian to die in his garden is like hitting the lottery

The GOAT of movies. The casting acting director score screenplay script lighting set design. Just a Masterpiece

I often read a lot of comments that talk about how accurate Michael's shots were in the restaurant. Michael was not involved in mafia affairs at that time, but he was a hero of World War II. He probably shot better than anyone in his family.

The baby at the baptism that was supposed to be a boy was actually a girl, she was Sofia Coppola, the director's daughter who is an award-winning director herself today.

The Oscar on the table in producer's room was genuine. While filming this movie Coppola had won the Oscar for best adapted screenplay for PATTON (highly recommended, BTW) and the statue was sent out to him so he put it in the scene.

In the book, Michael and Appalonias courtship is really sweet and wholesome.

A hidden gem is Tender Mercies, for which Duvall won Best actor. He sings in it. Horton Foote won Best original screen play. Foote won an Oscar for To Kill a Mockingbird, which was Duvalls first movie.

Veterans often wore their uniforms (especially for a formal event like a wedding) right after returning from overseas, but switched to civilian clothes soon after. The year in the beginning of the film is 1945/46.

“Take the gun ,leave the cannoli “ wasn’t in the script. Richard Castellano just said it . They left it in.

McClusky: "You gotta go, you gotta go" Christina: "This putz"  . Love it--perfectly said! I am a Second-generation Bronx Sicilian with a Jewish grandmother----I grew up hearing Sicilian dialect Italian and Yiddish all mixed together. I tried to convince a Neapolitan girl in my Catholic school that "Chutzpah" was Sicilian. True story.

I've probably watched a hundred reactions to this movie and I found your ability to keep track of who's who was impressive. So many others get lost at times. Good job! Keep that in mind for Part 2. In my humble opinion, The Godfather is the greatest movie ever made. And if it isn't, Part 2 is. Both perfect 10's in every category... director, casting, acting, cinematography, lighting, music... just everything. Part 3 not so much but still good enough and worthy of a watch to complete the trilogy. But you'll absolutely love Part 2!!!!

The horse head scene is not what "go to the mattresses" means.  The phrase originated (most likely) during the violent mafia conflicts of the 30s and meant that they were preparing for a battle.  Before a conflict would begin, assassins would be stationed in empty warehouses, apartments, etc., with basically just mattresses on the floor, until they were called into action.

According to the novel,, the reason the sentences of the two boys who beat up the undertaker's daughter were suspended was because they came from rich influencial families. And they didn't "smile" at him. They quickly walked past him out of the court room without giving him a look.

The wedding at the beginning of the movie is late 1945, the end of WW2.  
Michael is wearing a US Marine Corps officer's uniform at the wedding.  His rank (metal badges on his shoulder epaulets) is Captain, indicating that he commanded a company of 3-4 platoons totaling 150-200 men.  When he enlisted, because he was a university graduate, after basic training he would have been sent to Officer Candidate School and commissioned a 2nd Lieutenant, commanding a platoon of 35-50 men.  To be a Captain he has to have been promoted twice during wartime, which means promotions for merit/performance, not just because he was in for a certain length of time.

The ribbons above his left breast pocket indicate medals he has been awarded; the most important are at the top and rows are organized left of viewer (center of body) to right of viewer (arm side).
He has a Silver Star (3rd highest for combat valor with serious risk to own life), a Navy & Marine Corps Merit medal (highest for non-combat life saving with serious risk to own life), a Purple Heart (wounded in combat), and a Pacific Theater Campaign Medal with 2 Stars (indicating he participated in 3 Pacific Island battles).
He has more combat experience than any of the older mafioso who laugh at him wanting to kill Sollozzo and the police captain, and usually more experience than everyone else in the room put together.  Note also that he refers to the pistol as a "weapon", not a "gun", reflecting his Marine training.

Oranges play an important role, every time you see oranges a death or a very important event is about to occur. 😊

Now you know how Frank Sinatra got the part in "From Here to Eternity".

The singer was based on Frank Sinatra.  The story about the band leader was supposed to have happened early in Sinatra's career.  The part about the movie is based on rumors about how he got a part in the real movie "From Here To Eternity" (1953), which re-ignited his career.