For any simile, metaphor, or personification, there is a subject and an object.
The subject is the thing that is being described. It is usually something that actually exists in the story (frequently a person, place, thing, idea, or feeling). The figurative language leaves the reader with a better understanding of the subject.
The object is the thing that doesn't necessarily exist in the story, and is being used as a comparison to the subject. In other words, the object is the thing that the subject is being compared to.
The overlap between the subject and object of a simile, metaphor, or personification is what the two things have in common. This is how figurative language works: there is usually quite a bit that the subject and object have in common, and the simile, metaphor, or personification can communicate quite a lot to the reader, while only using a few words.
In the opening chapter of Of Mice and Men, as the narrator describes the scene where George and Lennie end up camping for the night; the narrator mentions:
"On the sand banks the rabbits sat as quietly as little gray, sculptured stones" (Steinbeck 2).
Subject: rabbits (sitting on the bank)
Object: little gray, sculptured stones
To understand the overlap, we need to think about what the rabbits and the sculptured stones have in common. Here are some ideas:
Stillness: the rabbits are sitting quietly, so as not to be noticed by predators, and sculptured stones would be similarly unmoving.
Beauty: the fact that the stones are described as "sculptured" suggests that effort was spent to improve their appearance; while it's unlikely that somebody spent effort to improve the appearance of wild rabbits, it's still fair to assume that they have the kind of natural beauty seen in nature.
Color: This may seem a little obvious, but it is a trick used by many writers. There are practically infinite varieties of the color gray, so if the rabbits were described as "gray," then readers may end up imagining the color differently than what the author had imagined. While there is still great variety in the type of gray seen in stones, it is a more limited variety, so the author's imagination may be coming through a little more clearly.
In act 2, scene 2 of Romeo and Juliet (the famous "balcony" scene), while Romeo professes his love for Juliet and vows that he will be true to her, Juliet expresses that she's feeling a little uncomfortable with how quickly things are moving between them, and so she says:
"although I joy in thee,
I have no joy of this contract to-night:
It is too rash, too unadvised, too sudden;
Too like the lightning, which doth cease to be
Ere one can say 'It lightens'" (Shakespeare II.ii.126).
Subject: "this contract to-night" (in other words, Romeo's willingness and eagerness to swear vows of love)
Object: lightning (which, she mentions, is gone so quickly that it's always gone before someone can say that they've seen it)
Taking a look at the overlap, we need to think about what vows of love (for a relationship only hours old) has in common with lightning.
Brightness/visibility: lightning lights up the sky and the area around it, just like this newfound love seems to be a source of incredible brightness for the young lovers; both lightning and their love stand out in stark contrast to everything else around them.
Suddenness: lightning is unpredictable and instantaneous, and Romeo's desire to swear his love to Juliet is something she would never have predicted (unpredictable), and it is happening only hours after meeting her (instantaneous)
Awesomeness: (By this, I mean the old-school definition of "awesome": so imposing and impressive that it inspires fear, not the modern/slang definition of "interesting, impressive, or trendy.") Lightning is so incredibly powerful that it can destroy things that seem sturdy or protected; being nearby a lightning strike almost always scares people nearby. The newfound love between Romeo and Juliet seems so overwhelmingly powerful that Juliet is likely a little scared of the intensity of her feelings.
Destructiveness: As mentioned in the previous point: lightning has the power to destroy. Even if don't know the entire plot of Romeo and Juliet, even a minute into the play the audience knows that Romeo and Juliet will end up killing themselves, and those who know the entire story know that the destruction and death does not end there.
If you're reading this because you're studying for an upcoming test, understand this: your answers on the test do not need to be as detailed as the examples above. Rather than explaining each overlap, as I've done here, you would only need to list the ways in which they overlap. So, using the above examples, your answers might look like this:
Of Mice and Men: Both the rabbits and the sculptured stones are still, gray, and beautiful.
Romeo and Juliet: Both lightning and Romeo's vow of love are bright/obvious, sudden, awesome/intimidating, and destructive.
Your answers would not need to be any longer than that. However, you may end up earning only partial credit if you only identify one way in which the subject and object overlap, when there may be a variety of overlaps (and multiple overlaps should be easy to identify).