Now that his sitting for the Medical School Admissions Test (MCAT) is behind him, Castillo has the fall to focus on football and football alone.
Though he began studying for the 8-hour test a year and a half ago and took the exam during the grind of two-a-days, Castillo insisted that it was no sweat.
"The way I look at it is if I can play in front of 85,000 or 90,000 people and millions of people in front of TV, I can go take an exam," Castillo said.
The next test – FSU's season opener opposite Miami on Monday night – fits that description. It's an assessment that the Seminoles haven't passed in their last six attempts.
"I can always take the MCAT again," said Castillo, a Palm Beach Gardens native who grew up supporting the Hurricanes. "This is my last chance to beat Miami."
If all goes to plan, Castillo will attend the newly-opened Florida State College of Medicine starting next summer. His application package includes letters of recommendation from FSU football coach Bobby Bowden, university president T.K. Wetherell and half a dozen faculty members.
Also working in his favor is the multitude of time that Castillo has already spent in doctor's office. Injuries forced a pair of redshirts to begin his career and a nagging string of ailments has kept Castillo out of games (including last season's Miami game) ever since.
Since his aspiration is to become a surgeon, the dislocation of two his fingers late last season caused a serious concern that the amiable Castillo always laughed off.
"You're not a good doctor unless you can relate with your patients," Castillo said. "I will always be able to relate to my patients."
Castillo has been with the team longer than five of its assistant coaches and has snapped to a dozen different quarterbacks including practice.
Along with wide receiver Willie Reid, the other co-captain, Castillo will be called up on to provide stability for an offense permeated with inexperience.
"He's definitely the guy in the locker room. He knows that," guard Matt Meinrod said. "When there is something that needs to be said, he is going to be the guy that is going to stand up and say it."
Along with Meinrod, also a senior, Castillo will be charged with helping the reshuffled line remember their assignments as they try to pave lanes for the FSU running game. Fans will get a first look at new offensive line coach Mark McHale's NFL-style zone blocking tactics Monday night.
"David is a pleasure to be around and pleasure to coach," McHale said. "He's always eager to learn. He'll stop you in the halls between meetings and during lunch to ask questions about this play or that play or this play."
Much has been made of Miami's current run of six straight wins in the series but Castillo's personal misery runs much deeper. A Miami fan in high school, Castillo watched probation-ridden UM drop five straight to the Seminoles.
"We need to get this win under our belts," Castillo said. "That way I can forget about all that."