Nearly one year ago, Philando Castile was shot and killed during a traffic stop by Officer Jeronimo Yanez.

Tuesday, the trial begins.

The world learned of the shooting in real time because Castile's girlfriend, Diamond Reynolds, broadcasted it live on Facebook, and it's a case that has been debated in the court of public opinion ever since.

"I think there are real high expectations on both sides," said Marsh Halberg, a criminal defense attorney and former prosecutor.

Halberg says the ramifications of the case mean both sides are under considerable pressure to win.

"There is obviously more public pressure on the prosecution in this case. There's an expectation I think by some people in the community that we expect a conviction here that, if it doesn't happen here, when is it going to happen?" Halberg said. "On the defense, they have the pressure of defending a police officer. The first type of case like this in the state to go forward."

Halberg says it's important to remember that Ramsey County Attorney John Choi decided to charge Officer Yanez with second-degree manslaughter, not first-degree murder, meaning the jury will have one major task.

"You're going to focus on two words: Culpable Negligence," Halberg said. "You're going to go, what the heck does that mean? (Jurors) are going to be told that culpable negligence is gross negligence; a very high level of negligence, failure to have reasonable care; coupled with recklessness, just being reckless."

When Ramsey County Attorney John Choi announced charges against Yanez back in November, he also made it clear the jury will have to consider what was reasonable.

"No reasonable officer, knowing, seeing and hearing what Officer Yanez did at the time, would've used deadly force under these circumstances," Choi said.

"I think you're going to hear that phrase in opening statement and I'll bet you hear that phrase in closing statement," Halberg said. "The question we have here is… reasonable has to also be viewed from the standpoint of a reasonable officer. It's just not a reasonable person, it's the reasonable officer based on their training, their knowledge, their experience. And why seven shots?"

Halberg says officer Yanez and Diamond Reynolds will give very different arguments about what was or wasn't reasonable that night, which is why he says a verdict will come down to their credibility and the 12 people judging it.

"I think jury selection is the most important part of the case," Halberg said. "There are studies I've seen that say people decide how they're going to decide a case just minutes into a trial. I've tried cases that I knew I won before I started and I've lost cases I knew I was going to lose before I started just because of the makeup of the jury I had to work from."