This mixes with a serious Tearjerker. Peter describes the influence his late brother, Larry, had on Optimus Prime.
Peter Cullen: From the get go, from day one, you know, I had something to work with. I had the influence of a brother - a former Marine - I get to do my brother, because, you know, that's pretty much what he was, that hero, to me.
He was once asked to guest star as Optimus Prime in a Robot Chicken sketch. He declined, saying that he loved the role too much to make fun of it. The creators of Robot Chicken have stated how much they respect him for that.
According to DVD commentaries, Cullen was not informed that the Transformers Prime version of Optimus would survive the first arc of the third season until the recording session for "Rebellion" came along. Cullen apparently became depressed, worried that kids would go through the same trauma of Optimus dying as kids did in 1986 with the original movie. D'awww.