Some highlights:
- Optimus having lips was because despite best efforts by ILM, the face mask while talking "looked horrible - they didn't resonate".
- There was a little cheating in regards to car to robot mass ratio due to design and look. This also resulted in more complex design and transformations.
- Screen tests resulted in Sam killing Megatron rather then Optimus, for increased impact and bravery.
(source)
