As far as being "required", suspicions about the source of funds (cash deposits) and transactions having no known economic purpose or being out of pattern (cash withdrawals) are categories that might fit certain levels of cash deposit activity, and thus require certain levels of due diligence to mitigate a SAR filing. But not all cash activity will rise to this level, so you need to find a level that balances customer service (not asking too many questions) and your BSA/AML obligations.
I am not sure what levels of cash activity you are asking questions at. By asking about large cash withdrawals that are out of pattern for customers we have interrupted several fraud schemes recently. We have also run into some possible elder abuse situations when POAs or children who are joint owners managing parents accounts withdraw large sums of cash over time that they claim are for home health care payments (we ask them to start writing checks to the workers instead).
You will get a number of customers who tell you to mind your own business about cash withdrawals. The "preventing fraud" angle allows the branch to bring up the subject in a less accusatory manner ("have you been asked to send this money to an old friend, business partner, IRS, SSA, grandchild's lawyer, etc"). If they seem of sound mind, the source of funds is valid, and their reason isn't too wacky we tend to back off at this point.
Cash deposits, particularly to a personal account or a non-cash intensive business account, have a lower threshold for questioning in my opinion. There is a certain level of cash activity that would be expected (that will vary depending on your branch footprint and customer profile), but an unknown source of funds has a lower threshold for questioning than an unknown use of good funds, in my opinion. I don't know of a good way to have these conversations, but they can be necessary if you don't want to default to SAR filing every time a customer deposits a large amount of cash.