There were three actors cast to play Jason, Tristan Davis-Jung was the final choice.
There were three actors cast to play Jenikus, in fact, Tristan Davis-Jung was the original Jenikus. A scene was even shot and edited with him as Jenikus. It is a bonus feature on the DVD.
Many of the voice overs are actually director Zack Hosseini, as they had to make a tight deadline during post-production.