How deep must fat be inserted when performing a cheek fat transfer? How do you make sure that you won't damage a facial muscle?

Depth of fat injections is usually related to the purpose of the procedure. If cheek bone area enhancement is being considered and one wants to produce a modest uplift, a deeper injection is necessary into the facial compartment and muscular space to produce the desired effect. If a more limited and superficial correction is needed the fat can be injected into the subcutaneous space, sometimes accompanied with needle release if fibrous indentations are present. In any case fat injection consists of depositing fine threads of fat dispersed gently with a blunt tip needle. In my experience of several thousand fat injections, I've never encountered muscle damage. 
Christian Drehsen, MD 

This is an excellent question which further emphasizs the know how of doing this procedure well.  Over the 19 years I have had women asking to have restoration of their cheek with fat or even have cheks similar to Kim Kardashian or other celebrities they have liked.  This is only possible through proper injection technique and knowledge of facial structures and know how.   I have not seen a single adverse event with my injection and treatments of fat transfer and purpose here is not to teach others where and how to inject since that is a cumulitive knowledge and expereince of years.  Nevertheless, you should be rest assured that a well experienced and knowlwedgable doctor would put your mind at ease and worries when you go for consult.   Thanks for excellent question

Fat grafting (especially to the face) should be done in microdroplets with a blunt cannula. You are absolutely correct that there are very key structures in the face that would be devastating to harm. However, the likelihood of this goes down when small droplets are carefully placed with a cannula that is not sharp. Therefore, key structures such as nerves and arteries are less likely to be damaged and merely pushed aside. I like to put fat in all different layers in the face (deep, intramuscular and subcutaneous). Spreading it out between the different layers without overwhelming any one layer is best for the longevity of the fat cells and natural appearance.