Jackson Rathbone was asked why they didn’t get much into his backstory during New Moon, and he told TeenMag–
“Well it doesn’t really come out too much in the book. It’s one of those things were it’s understood but in the third book it really comes out like the back story and everything.
“I think one of the things we didn’t want to get too much into was focusing on all the other vampires.
“We wanted really to establish this relationship and how she (Bella) gets sucked into the world of the Cullen family and once she’s in the world that’s when things kind of start coming out.
“One of the things about Jasper is he doesn’t use the ability all the time. It’s something that he’s very strict about using because it’s like an invasion of privacy to make someone feel something, to manipulate emotions.”
--Kitzie
Source: http://www.newmoonmovie.org/2009/06/jackson-rathbone-looking-forward-to-eclipse/
