Lets look at Myrtle first. Fitzgerald never names her but describes her for a few pages. In fact his choice of words to introduce Myrtle Wilson were: "Tom Buchanan's mistress". He paints Tom as a little "tanked" that day and then Tom calls her "my girl" as if she is a possession. When Myrtle actually makes her entrance, as narrator, Nick uses these beautifully vivid adjectives to describe her: "thickish figure", "faintly stout", "she carried her surplus flesh sensuously", and "an immediately perceptible vitality about her". These descriptions serve as the sensory details of sight or at least imagery. They also give us the idea that she is a sexual being for Tom, but not the most petite cute little thing either.
Tom is painted as a forceful individual through character actions and dialogue. Nick describes Tom as having a "determination to have my company that bordered on violence." Later, Tom demands that Wilson hurry up and get the car ready that Tom wants to buy from him - this set of dialogue between the two demonstrates Tom's ability to assert himself to anyone. Finally, Tom says to Myrtle,
"I want to see you... Get on the next train."
That's a pretty demanding command from a man who isn't Myrtle's husband within feet of Myrtle's husband.
These vivid details help portray the physical nature of their relationship and they rely on language used figuratively in order to develop such great characters.