One Walter Brennan-Gabby Hayes kinda old bird (sheriff, postmaster, hermit, whatever) who is usually the operator of the ineffective six-shooter or thirty-ought-six.
High heels for the imperiled love interest. Maybe keds if she’s really one of the boys.
A radio (or rarely television) which interrupts regular programming for an up-to-the-minute news bulletin - and is then immediately turned off.
You left out an important element: teenagers making out in their car, out in the desert. They see the monster (or evidence of it), and hurry back to town . . . but nobody believes them. But one man does go to investigate . . . and doesn’t come back.
In the climax to the movie mentioned in the OP, the deadly mantis, trapped in a tunnel, flips over a bus. On the bottom of the bus, clearly visible is the embossed word, “Tonka.”
the 8-year old kid. Billy, Pepito, you name him. ALWAYS an irritating shrill-voiced little dipshit whose disobedience ALWAYS gets him trapped by the monster, and you ALWAYS root for the monster.
Well, except for The Blob. That kid was just dumb.
This was not a particular feature of fifties SF films, but a requirement of the Hays Code that governed all films in the 50s. From the Wiki on pre-Code Hollywood:
It took me no time at all to think of more exceptions to the “eight-year old kid” rule. I wish to amend my statement to:
Usually an irritating shrill-voiced little dipshit, whose disobedience usually gets him trapped by the monster, and you ALWAYS root for the monster.
Thank you for your understanding.
EDIT: I did just think of another often essential feature: Bronson Caves.