So I had to read through this a few times in order to understand it. If I'm correct, you're showing an old man who's lost his wife after many years of marriage.
I wouldn't say this is garbage as much as I'd say it's HARD TO UNDERSTAND.
In the beginning of the script, are these flashbacks the old man is having OR are you simply showing us their history? If you're showing us history, unfortunately, I think you'd need to tell us that with a title card but I am of the opinion, this could be done much better as a flashback of the old man.
And? If this is a flashback or you decide to rewrite it as one? I would begin with the old man coming in as your first scene... I'd also have the two employees see him walk in and whisper among themselves so we see that they know the guy but we do not know everything right up front.
I would then scratch the Diet Coke as you said you'd already do and to be honest? I'd scratch a soda too. There's just NOT enough enough visual play that can be done with a soda in my humble opinion. What about an ice cream soda? These are more reminiscent of the 50s anyway and give you more to play with i.e., he can take a spoon to steal some of her ice cream or he could even touch the ice cream and stick a little on her nose... All in the spirit of being "PLAYFUL."
Additionally, just on the face of what I've read? Is this old man suffering from dementia of some kind? It would seem so because if he shows up at this diner all time waiting for his wife to walk in why would he do that unless he's somehow forgotten that she's dead. Of course, I don't know if she's in fact dead because your script doesn't say. I have to connect the dots since she never shows up in all the times he's been waiting for her as an old man.
I think I get (kind of) what you're trying to accomplish here... It's a bit of a downer in my opinion because the woman never shows up and since we get no real backstory, I have to assume she is NEVER going to show up.
So yeah... It's actually a bit dark but I like dark but in this particular instance... While this is dark, it also doesn't seem to inspire any actual hope for the old man.
Which makes me wonder if that's the message here assuming this story does in fact contain a message i.e., that we all grow old and we lose people along the way that NEVER come back.
I for one am already LIVING THAT LIFE and am well aware that it exists... LOL. I'm 65 years old and people -- family -- friends -- and associates -- are dropping like flies each and every month. I am the last one alive in my immediate family. Do I want to watch a short that reminds me of that?
Not really.
But I wouldn't mind watching a short that gives me some inspiration to KEEP GOING, meet new people and find new inspiration to carry on because quite frankly, until science figures out a way to keep us all alive? Aren't we all going to go through some version of this even without some kind of dementia?
I know I do. I am constantly reliving fond memories with those who are gone and I don't think I'm suffering from any dementia yet...
So now to the writing... THERE IS A HELL OF A LOT OF OVERWRITING HERE. Which is fine. It's your first attempt at something like this so it makes sense. As I've mentioned elsewhere here on the forum, I highly recommend the following book if you really want a streamlined version of how to write a screenplay that doesn't break all the unwritten rules:
Screenwriting for Neurotics: A Beginner's Guide to Writing a Feature-Length Screenplay from Start to Finish
By overwriting, let me show you what I mean...
I am not going to do this for the entire script because I don't have time but you'll need to tweak and polish all the way through what you've written.
You wrote:
INT. FAST FOOD RESTAURANT
The scene is set in the 1950's. A young woman in her early
twenties with dark hair and freckles sits in front of a
pitcher window in a booth of a small diner. She slurps her
Diet Coke and stares at the empty booths in front of her.
First you call it a fast food restaurant and then you call it a diner. BE CONSISTENT.
I'm certainly not saying to change it to the following but this is really all you need -- especially if this is going to be a short.
FLASHBACK TO INT. DINER - 50S - DAY
A young woman (20s) sits in a window booth, slurping
an ICE CREAM SODA.
There are any number of ways to format flashback scenes... This is just one way. Another could be:
FLASHBACK
INT. DINER - 50S - DAY
A young woman (20s) sits in a window booth, slurping
an ICE CREAM SODA.
You could just as easily use:
FLASHBACK TO
FLASH ON
There is no real rule here in HOW to format it... Just be consistent and be absolutely CLEAR so it doesn't slow down the reading. Again, I woudn't begin the story/script with a flashback. I'd begin with the old guy coming into the diner and then once he's sitting in his booth and got the ice cream soda? Use some kind of a trigger to trigger the flashback. Maybe he touches the ice cream with an index finger. Maybe he steals the cherry off the top.
Make sense?
I would also read up on how to write flashback scenes so it all makes sense:
How To Write Flashbacks in Screenplays (With Examples!)
HOW TO WRITE A FLASHBACK IN A SCRIPT LIKE A PRO SCREENWRITER.
How To Format A Flashback Like A Pro Screenwriter
All About Flashbacks
HOW TO WRITE A FLASHBACK IN A SCREENPLAY
As you can see from all the articles? There is no ONE WAY to write a flashback. Just be absolutely CLEAR that that is in fact what you're doing.
Good luck!