After receiving thousands (trust me, thousands) of resumes, they selected a group of little girls to audition for the role. Also, many young girls (mostly models & little actresses) from important Talent agencies were requested to audition as Renesmee. Those lucky girls had the chance to read a couple of lines from the Breaking Dawn script (a draft of the original script) and say the lines in front of the cameras.
Once Summit had their “favourite” actresses, the girls were requested to be seen by Bill Condon (the Breaking Dawn director) and mr. Condon was the one who had the last word, of course.
In addition, you can watch this audition video of another young actress, Tilly Cheri, who auditioned for the role. Sounds like the lines might be from the script? Yeah? You never know, but it’s fun to think so.