Harper, who arrived at Clemson two years ago out of Sequoyah High School (Canton, Ga.), has had to battle his way up the depth chart one practice at a time.
Last year, he established himself as the third string quarterback behind Proctor and starter Charlie Whitehurst and actually saw the field in the Duke game, completing 1-of-2 passes for 15 yards.
This year, he finds himself in a position that should the unthinkable happen to Proctor, he'll be the guy called upon to run Clemson's offense.
"It's not all that different this year because I've been preparing, working hard and doing everything it takes to be a successful quarterback," he said. "But the number of reps I'm getting is a lot more. The reps just weren't available before. Now, this year they are and I'm trying to make every opportunity of it I can."
But the battle for the back up position only tells part of the story.
Sure, Harper wants to maintain that edge over fellow redshirt sophomore Tribble Reese for the No. 2 spot, but he is also well aware of what will be in store for him next spring.
With Proctor finishing up his eligibility this fall, the starting quarterback position at Clemson will be wide open. Competing for the job will be Harper, Reese, Michael Wade and current Clemson verbal commitment Willy Korn.
Harper says he's ready for whatever happens.
"The only thing I can worry about is working as hard as I can and letting everything take care of itself. I'm not going to let up at all. I'm a big competitor and I'm not going to back down," he said.
Keep in mind; Harper has faced this kind of talk before.
When the Tigers signed Harper and Reese in the 2004 recruiting class, many thought that Reese would end up becoming the starter before Harper and even Will Proctor.
All the while Harper remained under the radar, continuing to work on his game.
"I've gotten bigger, stronger since I've been here," he said. "But I can definitely run. I can get out on the edge and get up field if I'm under pressure. I've been working on all aspects of the game. I came here to play football. That's what I want to do."
This spring, some of Harper's hard work finally began to pay off more in the public spotlight, as he completed 8-of-9 passes for 63 yards and a touchdown in the final scrimmage before the Orange & White game.
And so far during fall practice, Tommy Bowden has given the edge to Harper as the team's back up quarterback.
"Cullen is just a little bit ahead physically. He's gotten just a little more work," he said. "And physically, Cullen is just a little bit bigger. A little bit stronger than what Tribble would be if you are talking about physical attributes."
So for now, Harper remains the back up, ready to take the field if needed.
The battle to become the starter will have to wait until the spring.
"Coach Spence just keeps reiterating the fact that we are all one play away from seeing the field," he said. "You can't expect Will to be in there for every snap.
"You have to be ready."