McMullen had been the backup to Steve Bellisari, who was suspended indefinitely Friday after being arrested for drunken driving. He will make his first start Saturday at home against Big Ten co-leader Illinois.
"Scott's biggest attribute is that he's an outstanding young man in pressure situations," said Mike Hensley, who coached him for four seasons at Granville High School, 25 miles east of Columbus. "He doesn't get excited and that's what makes him so good."
Granville's athletic director, Tim Dennison, also said McMullen was noted in high school for his calm demeanor, no matter what the situation.
"He's just very cool, collected and unimposing," Dennison said. "If you had a player you'd want to lead you in a tight spot, he'd be it. What's unusual about him is that in the biggest moments, he'd look like he was asleep, and really, it was just that he was relaxed and happy to be in a situation where he had a chance to do something big."
McMullen threw for 67 touchdowns, a state record at the time, at Granville and showed some of that calmness early in his career.
Hensley said McMullen took over for a struggling senior quarterback in the middle of the 1995 season and, in his first starting appearance, led the Aces to a victory in which they came from 11 points behind with a minute-and-a-half to play.
"I knew then that he was someone special, and coaching him was a privilege from then on," Hensley said.
"He may not be a vocal leader, but he always led by his performance," said Dennison's son, Cooper, a high school teammate of McMullen's.
"He wasn't a very good practice player, but he was a clutch performer in high school and I feel good that he'll come through on Saturday," Dennison said from his dorm room at Miami University in Oxford, where he is a junior studying medicine.
Cooper Dennison said he and another friend from Miami have tickets for Saturday's game and had made plans earlier to visit McMullen on Sunday to help him celebrate his 21st birthday.
Hensley, who talked with McMullen on Friday, said the quarterback didn't know about what had happened to Bellisari until the news had spread throughout the campus and the community.
"He was in class all morning Friday and came back to his room to find about 15 messages on his answering machine, and that's when he found out," he said.
"My conversation with him was limited, but he sounds fired up and ready to go, and I'm going over to campus tonight to meet with him and help settle him down a bit.
"Sometimes kids step up when it's least expected, and I'm hoping that's what will happen with Scott. My biggest concern is that he will be rusty because he's seen so little action the last two years. His last start was in 1998 against Millersport, and now he's going to face Illinois on national television. It's a big difference."