Every day, they convened for one-on-one practices. This was what Dad knew best, so this was their relationship.
"Football is what he grew up with, that's what he knows," said Greg, who starred at St. Joseph's Prep in high school. "So that's the way we spent time together. That's the way we bonded. We went and worked out."
Juan does have a defensive background that comes in handy with his son. At Texas A & M-Kingsville and with the USFL's San Antonio Gunslingers, he played linebacker. And considering Juan's wife, Zaida, was a gymnast, Greg naturally evolved into a prototypical cornerback.
The unusual sleeping routine has roots in Jon Gruden's tenure with the Eagles as offensive coordinator in the mid-1990s. As a quality control coach - or as Castillo called it, Gruden's "errand boy" - he needed to be at work at 4 a.m. Gruden was a workaholic, so Castillo went along for the ride.
"What ended up happening, as my kids got older, I found out that this was a good way to get work done," Castillo said. "When Coach lets you go then you feel like you can go home and you've done all your work."
As much as Dad's valiant insomnia inspired his son, Juan vows motivation has been a two-way street. In addition to Greg earning a scholarship, younger brother John is a runner at North Carolina State. After an injury-ravaged freshman year, John Castillo moved to Texas this summer. Living with his grandparents, he's training to come back stronger next spring. The Castillos also have two younger sons, Andres and Antonio.
This is what drives Juan every morning. This is what tells Juan he has done something right over the years.
"I see my sons like that and it motivates me," Juan Castillo said. "I see my son in Texas giving up his friends and just going away. He's just down there training, trying to get better so he can compete.
"It's easy for all of us when things are going well but when things aren't right, what are you going to do?"
Greg Castillo has no clue how many days he has spent with Dad, no idea how much backpedalling he's done. Through the exhausting workouts at Lenape and the NovaCare Complex indoor field, Juan was both calm and assertive. In his free time, as fleeting as it was, Dad studied up on the cornerback position and broke down Greg's game tape.
The next day, they instantly cleaned up all mistakes on the practice field. Every scant blemish was fixed. Now, Greg is a Big Ten cornerback.
Castillo tries to apply such individual attention to his day job. The goal, he insists, is for every lineman on his team to end up on a NFL roster every season. Somewhere. So after every OTA practice, there's Castillo. Grabbing waiver-wire-bound linemen by the jersey for 15, 20, 25 minutes after practice has ended.
Hand placement, foot movement, off-the-ball surge, everything is analyzed. It's a totally different position, but some coaching methods transcend positions.
"He's big on technique and repetition," Greg said. "When we practice, it's about muscle memory as a corner. If you keep doing it over and over again, it becomes natural."
Training, day after day, was the vehicle that brought Juan and Greg Castillo together. Sometimes, a simple message is lost in the shuffle. Slouched on a couch inside an office room at the NovaCare Complex, Juan Castillo gets choked up and fights back tears that trickle out for a brief moment.
"As a father," he said, smiling, "you just wish your sons would say that they love you more often."
Unprompted, Greg made it clear. Looking back on those evenings on the field, he realizes he owes everything to his father.
"Even though I act like I don't appreciate the things he does, I really do appreciate everything he does," Greg said. "I love him so much."