Here is a good article on how Harris's success is not simply years of backup experience but on dedication to improving his weaknesses.

Anyone who has ever seen Trevor Harris go through his pre-game routine will see how he spends time working on his footwork, the foundation for every throw a quarterback attempts.
At times, Harris’ footwork is off, leading to errant throws. It’s a flaw that led to one of the two interceptions he threw in Ottawa during their most recent game when the Argos earned a well-deserved and big-time win.
“What we like him to do, and you can’t do it all the time, is what we call tracking the receiver with your body,’’ began Argos head coach Scott Milanovich, a one-time quarterback. “If you fight your body and your hips, it makes you less accurate. The best of them (quarterbacks) do it and it’s something he (Harris) continues to work on.”
Every once in a while during practice, Harris is picked off when his fundamentals are poor.
In Ottawa, RedBlacks linebacker Damaso Munoz produced a pick six off Harris for just that reason.
“He was off his spot,’’ said Milanovich. “He was kind of in the middle of running and throwing.”
What Milanovich now requires from Harris is to be clean which, in football, means protecting the football and making good decisions.
“He has to eliminate turnovers,’’ said the coach. “I like to see him with a clean scorecard.”
http://www.torontosun.com/2015/10/03...o-keep-in-step