Securing effective patent rights is all about timing and information. The first patent application must contain enough information to permit a person of ordinary skill in the field to put the invention into practice. But that information must be novel when the patent application is filed.
It follows that you need to have enough information to enable the invention when you see your patent attorney. It won't be very useful if you spend the money filing the application only to find out later on that the patent specification does not properly enable the invention.
It follows that you may need to go so far as building a prototype or having a prototype built before seeing your patent attorney. Then, once the patent application is filed, you need to be sure that you keep your patent attorney apprised of any changes or developments to ensure that those are covered before they are disclosed without an obligation of confidentiality.
Note that you should still speak to your patent attorney as soon as possible. Your patent attorney may be able to prepare a non-disclosure agreement that you can use when speaking to product development experts.