Kauser, a St. Anthony police officer had come to assist his colleague, Jeronimo Yanez, in Falcon Heights, Minnesota, last summer. Kauser stood at the passenger's side of the car as Yanez spoke with Castile through the driver's side.
Castile was not threatening from what he could hear of their conversation, Kauser testified Tuesday in the manslaughter trial of Yanez, his longtime friend.
But Kauser said he didn't hear most of their conversation because he was keeping an eye on the passengers -- Castile's girlfriend, Diamond Reynolds,
in the passenger's seat and her 4-year-old daughter in the backseat.
The dashcam video showed Kauser with his thumbs hooked in a casual position on his bullet resistant vest.
"I didn't feel threatened at that point from where I was standing," he said.
When shots rang out, Kauser said he was "surprised" because he didn't know there was a gun in the car and he hadn't seen Yanez pull his weapon.
Kauser also said he never saw Castile's gun.
When asked if Yanez followed protocol during the entire incident, Kauser replied: "I think he did what he was supposed to do in that situation."
Traffic stop turns deadly
Officer Yanez is charged with second-degree manslaughter, plus two felony counts of dangerous discharge of a firearm, in the shooting that fueled more nationwide protests over the excessive use of force against black people.
On July 6, Yanez pulled over Castile, saying he looked similar to a robbery suspect.
Castile told the officer he had a weapon on him. Yanez told him not to reach for it. Seconds later, Yanez opened fire.
Castile died, insisting that he hadn't been reaching for his handgun.
He had a permit to carry a firearm in his wallet. His girlfriend said he had been reaching for his ID in his back pocket when he was shot.
The gun remains a critical component in the trial.
Yanez's defense says Castile ignored the officer's commands and reached for his gun when he was instructed not to do so. Prosecutors say Yanez acted negligently in using deadly force and had not given clear instructions.
Prosecutors tried to show Yanez was negligent by questioning Kauser and another police officer on how they would handle a situation if a person told them they had a gun.
"I ask where it is," Kauser said. He added he would then tell them to "keep their hands on the wheel or through the steering wheel on the dashboard."
Yanez did not give such instructions though.
The defense said Yanez didn't have the time to do so, because the situation escalated so quickly.
Where was Castile's gun after the shooting?
Also, two witnesses gave conflicting testimonies regarding Castile's gun.
Officer Juan Toran of the Roseville Police Department who had arrived on scene, gave CPR to Castile for about two minutes, he said.
As emergency responders prepared to place Castile on a medical backboard, they rolled him to his side when Toran said he saw a gun sliding out of Castile's right front pocket.
"Gun," Toran said he yelled. He removed the gun and placed it on the ground.
But the next witness, Eric Torgerson, a medic and firefighter for the St. Paul Fire Department, gave a different account.
Torgerson said he saw an officer "reach deep into Castile's pocket" to pull out a gun. He said the weapon wasn't sliding out.