Childhood Apraxia of Speech is a neurological disorder of the motor planning of speech. Here is one small example of how complicated that motor planning can be.
You might think that a /b/ is a /b/ is a /b/. Or you may have thought, correctly, that making a /b/ at the beginning of a word is different than making a /b/ in the middle or at the end of a word. But it gets even more complicated than that.
Say the word "book" five times in a row out loud, but before you say it the last time, freeze your mouth in the position it is in when you are about to make the /b/ sound. Your mouth should be pursed a little, almost like you're about to give someone a kiss.
Now say the word "bee" five times in a row out loud. Again, stop before you say it the last time freezing your mouth in the position it is in when you are about to make the /b/ sound. Your lips should be pressed together, almost like you just put on chapstick or lipstick and are spreading it evenly around.
Even though the words "book" and "bee" both begin with the /b/ sound, the motor planning for producing the /b/ is very different. For the first /b/ in the word "book," the motor planning involves the muscle motions necessary for lip rounding (because the following vowel is a rounded vowel). For the second /b/ in the word "bee," the motor planning involves the muscle motions necessary for lip spreading (because the following vowel is a vowel that involves lip spreading).
The difference between those two initial /b/ sounds is just one small example of how complicated motor planning really is. I just thought the example was interesting and I wanted to share.