The relationship between Spenser and Susan Silverman is one of the most enduring relationships in literature. They've been together for 35 years since 1974 when God Save The Child was published. Here's the scene where it began.
"Why don't you take off your coat?" she said.
"Well, it's supposed to make me look taller," I said.
"No, I guess not," I said and stood up and took it off.
"I don't think you need to look taller, Mr. Spenser," she said. When she smiled, the color of her face seemed to heighten. "How tall are you?"
"Six one," I said.
"Really? That's surprising. I must admit you don't look that tall."
"Even with the raincoat?" I said.
"Even with that," she said. "You're so wide. Do you work with weights?"
"Yeah, some. How could you tell? Your husband lift?"
"Ex-husband," she said. "Yes, he played tackle for Harvard and stayed with the weights afterward."
Robert B. Parker, God Save the Child.
Notice that Susan has six turns to speak and Spenser has five. Susan utters 58 words; Spenser utters 27. Susan's extra turn, which has 13 words, doesn't account for the difference in total words. Susan says an average of 9.7 words each time she speaks compared to Spenser's 5.4.
We're not going to fall into the old stereotype that women talk more than men. We want to know where the extra words come from. We can see that Susan's speech is more complex than Spenser's. Remember, both of them are very literate people. Spenser reads books that most people would never pick up. Nevertheless, Susan utters eight complete sentences and four fragments compared to Spenser's three complete sentences and four fragments. Correct grammar simply uses more words than incorrect. So one difference between men and women is that women's speech tends to have more complete sentences than men's. Note that both men and women pepper their dialogue with sentence fragments. The difference is simply in the ratio of sentences to fragments--two-thirds of Susan's utterances are complete, whereas only half of Spenser's are complete. So to make your character's speech more masculine, spice it with more fragments. To make it more feminine, check the grammar.
The "do drop." English speakers commonly drop the "do" or "did" from certain kinds of sentences ([Do] You want to get something to eat?) This is another gender marker. Susan retains it when she asks if Spenser works with weights. Spenser drops it in his answer. While both men and women commonly drop the "do," men drop it more often than women.
Women use language to connect, men use language to communicate. Certainly men connect, too, just as women communicate, but women's speech contains more connectors. When Susan says, "I don't think. . ." she is making a connection between herself and Spenser. She could have said, "You don't need to look taller," which would have been straightforward communication and is the way Spenser probably would have said it. Susan makes another connection when she says, "I must admit. . ." Phrases such as "I think," "I believe," "I feel" preceding a statement of fact serve to connect the speaker to the hearer. Such phrases are not exclusively female, but occur with greater frequency in women's speech than in men's.
The art of seduction. Man-to-woman speech is often more wordy than man-to-man. A man seduces a woman by telling stories and jokes about himself. Remember when Eddie Valiant, the detective in Who Framed Roger Rabbit? asks Jessica why she stays with Roger? She answered, "He makes me laugh." I'll bet our caveman ancestors did not club fair cave maiden on the head to get her onto the animal skins. Instead, he probably spun a tale about the hunt and maybe tossed in a joke about stepping in the mastodon dung. Spenser makes a play for Susan with a joke on himself about the coat making him look taller. It earns him a smile from Susan.
There are, of course, no absolutes. You can find men who never utter a sentence fragment (Barak Obama?) and women who always drop the "do." How your characters speak is part of their personality and the more you understand their personality, the more authentic their speech will sound. However, counting words, fragments, "do's," and qualifying phrases can help you add more feminine or more masculine attributes to your dialogue.
For more on this topic, see my Hawaiian-eye blog.