Hey there,
It really depends on the kind of person you are. As Jon mentioned, if you're naturally eloquent in interview-esque situations, then you can probably streamline the preparation a bit. When you do run through your mock interviews, try to find someone with two criteria.
For one, make sure they know interviewing. If you work with an admissions consulting firm, you won't have to worry about that. But if you run through family and friends, they may never have conducted an interview in their lives. Which isn't the right audience then.
Secondly, try to have at least one of the interviews be with someone who isn't familiar with your story. When we conduct our mock interviews, we pair clients with partners who weren't a part of their prep process for the first interview. That way, we can hone in quickly on what is and is not coming across and overall strength of the experience. So when we start building on / tweaking those responses, we have candid, objective feedback to work from.
Hope that helps!
Bhavik