Eddie Ray Routh allegedly shot Kyle and Littlefield at a gun range in Erath County two years ago.
Kyle's fame has skyrocketed in recent weeks because of the success of the movie "American Sniper," which is based on his life.
He was the most lethal sniper in American military history.
After he retired in 2012, he spent part of his civilian life trying to help other veterans. That's why he was at the gun range with Littlefield and Routh.
Routh's attorney said his client was suffering from post-traumatic stress disorder. He plans to pursue an insanity defense.
Prosecutors and defense lawyers started with a jury pool of about 800 people – 400 Thursday and 400 Friday. Of those expected Thursday, only about half showed up, but 150 of those were qualified.
They'll return Monday, where they'll be questioned by attorneys, and that's what will determine who ends up on the jury.
Potential jurors may be asked if they've seen "American Sniper" or read Kyle's autobiography, but those who have will not automatically be disqualified.
Mark Marett says he was excused from the list of potential jurors for medical reasons.
He's a big fan of Kyle.
"He's a hero and he was trying to help others, and the wrong thing went bad," said Marett.
Chris Conway says he was excused as a potential juror because he's already made up his mind.
"I don't really buy the insanity position in that," said Conway.
Opening statements are expected to begin next week. The trial should last about two weeks.
It's the largest trial ever held at the Erath County courthouse in Stephenville.
Prosecutors want a life sentence without parole -- not the death penalty.
The trial is taking place under a heavy net of security. Two streets are blocked off next to the courthouse, and there are officers at each corner. Additionally, new security cameras provide a view of the area.