Sam Witwicky, or "Spike" to his friends and family, is your average twenty-something living at home with his aging father and helping out with the family business. When giant robots land at his doorstep and make friends with him, he sympathizes with their plight and agrees to help them. While he's now the target of the Government and other giant robots, he at least got a free car out of the deal and a new best friend, so, you win some you lose some. 

Sam is the conduit to which the audience experiences the world of the Autobots and Decepticons, much in the way many of the human characters in Transformers are. He serves as a dose of reality in a sci-fi action adventure. I'm very much open to interpretation on this character's voice, so feel free to experiment. It just needs to sound authentically human, which shouldn't be too hard in a world of robots, right?