Castillo was the Eagles' offensive line coach when Johnson was running the defense. They spent countless hours and God knows how many gallons of coffee discussing protections and blitzes and family and a lot more.
"Jim and I had a special relationship," Castillo said. "I call Mrs. [Vicki] Johnson before every game. Sometimes we don't get to talk and I'll just leave a message. But she emails me, and I got to see her [Sunday] and give her a big hug."
Even though he's been gone for almost 2 1/2 years, Johnson still casts a huge shadow over the Eagles' defense and the men who run it. It was too big for Sean McDermott, who was fired after just two seasons. Now it's Castillo's turn. Only time will tell whether it's too big for him as well.
Many of us have been quick to pass judgment on him after his unit's horrible start, which included blowing three fourth-quarter leads and getting torched for 132 points in the first five games. "In over his head," has been an oft-used phrase by critics.
We have shaken our heads when he has urged patience, even as the Eagles' season seemed to be slipping away.
"We've started off like this before, and we believe in our plan and we believe in fundamentals," he told a skeptical audience of reporters 3 weeks ago. "We believe in the way we work. We understand that and we know that every week we're going to get better and better and better. That's the way it's happened here before."
There still is a lot of the season to be played, but for the moment, Castillo's unit appears to have stabilized. The Eagles have given up just 20 points in their last two games, marking the first time they've held back-to-back opponents to less than 14 points since 2008, and the first time they've done it in back-to-back wins since '04.
In the last 25 opponent possessions dating back to the second half of their Week 5 loss to Buffalo, the defense has given up just 23 points, held opposing quarterbacks to a 50.6 passer rating and allowed just 4.2 yards per carry after allowing 5.3 per carry in the first 54 possessions.
"On defense, we've got quite a few new faces," Castillo said. "So, it's almost like we were putting in a whole new defense. Even though some of the verbiage was the same, a lot of things were new. Plus, guys just needed to get to know each other."
The Eagles have five new faces playing key roles on defense - cornerbacks Nnamdi Asomugha and Dominique Rodgers-Cromartie, end Jason Babin, tackle Cullen Jenkins and rookie linebacker Brian Rolle. If there had been a normal offseason, that wouldn't have been a particularly big deal. But the 4 1/2-month lockout made it a big deal.
"It's been tough for Juan," Asomugha said. "First of all, he's never been a defensive coordinator before. And then he gets all this new talent. And it's like, 'OK, now we have to cater to what Babin does best and what Dominique and I do best.'
"You just can't do it. It's tough. So we've all had to learn how to play within the defense. It's challenged us, but it's made us better. We still have to keep it going. We're happy we got two straight [wins], but it's a long season ahead of us."
The lockout has forced everyone on defense, including Castillo, to learn on the run. He has tried some interesting things to hasten the getting-to-know-you process and build trust and communication among his players.
At the beginning of every defensive meeting, a player goes up to the front of the room and talks about himself and a hardship he has had to deal with in his life. Yes, it sounds a little summer-campish, but both Castillo and his players insist it has been beneficial.
"It's so the rest of the guys can get to know them," he said. "I mean, they didn't really know each other because they didn't have time to spend together like you would in a normal offseason. So that's been important.
"Now, all of a sudden, Nnam knows about Dominique and Dominique knows about Asante [Samuel] and Asante knows about Cullen [Jenkins] and Cullen knows about Trent [Cole]. It's been an awesome thing."
What has helped more than anything has just been playing together.
"We're starting to gel a little bit," Samuel said after the win over the Cowboys.
"The fact that we're further into it and we're all getting a little more comfortable with the defense, we're able to show what we can do," said Asomugha, who had his second interception of the season against the Cowboys and along with Samuel, Rodgers-Cromartie, Joselio Hanson and Jamar Chaney, helped hold Jason Witten, Dez Bryant and Miles Austin to a collective 10 harmless catches and 83 yards.
"We're not thinking as much. We've repped it so many times. We're doing the stuff we've been doing pretty much all year. It's just that now, we have a better grasp of it and we're making up for the time we missed in the offseason. Time has passed and we're all starting to get more and more comfortable with our roles."
Castillo certainly has made some mistakes. But he appears to have learned from them. He and defensive line coach Jim Washburn are more in sync as far as when and when not to use the wide-nine than they were when the season started.
It took him a while to figure out who his best three linebackers were and where they should be playing. But he eventually got it right. He tried to do too much too soon with Asomugha. But he quickly backed off the throttle, and now seems to have a much better feel for how to use both he and Rodgers-Cromartie.
"He's developed as much as everyone else has developed," Asomugha said of Castillo. "All of us, including him, were learning. He's never done this before. It's a tall task to do it here where so much is expected of him. He's grown and he's going to continue to grow.
"We're still making mistakes, and I'm sure he'll tell you he's still making mistakes. But we've played well the last two games, and that's always a positive sign and something he can build off of."