Listen in to a real conversation that you are not involved in and note just how much extraneous detail there is, how much fluff. Record a minute or two of it (tell the speakers, I should add, that you are doing so, or at least ask them afterwards if they are happy for you to use their conversation for this exercise). Now transcribe the conversation, word for word, grunt for grunt. Every little sound.
Imagine this transcript on the page of a novel. More than likely, there will be instances of speakers talking over each other, repetition (either of what one person has already said, or of what somebody else has already said), hesitations, ums and ers, fillers, veering off topic…
In addition to this fluff will be all of the paralinguistic features of body language that cannot be transcribed directly by the dialogue. These things, though, are rich territory for the narrative description that might go around and between your dialogue. What is more – if you have created characters that engage the reader, that the reader can form their own imaginative sense of, then readers will quite naturally imagine these things for themselves, without the assistance of the author.
So now take your transcript of real speech and turn it into a passage of fiction. Think about how to convert the speakers into characters on the page. Use the tips above to make the dialogue appear real, even though it is a fictional construction, and think about the balance of speech and narrative description, creating detail, meaning and pause from the setting, as well as giving the subtle suggestion of how the words are being spoken.