Not able to give more details as Bay has seriously chomped down on information leaks (dang it). Ferrar really liked Bumblebee, in part because he was built first and also a challenge since no lines had to use emotions with the eyes etc for the acting. The learning experience with Bumblebee then helped them in creating the other Transformers for the movie.
The full video is here.
In addition, there is a brief interview with 4 other members of the Transformers Visual Effects team here.
(source)