Perry Fewell attached the usual caveats to all the things he thinks the Giants defense can accomplish this season. They can be great, if they work hard enough. They can change games, if they grow comfortable with new schemes and new teammates. The fifth-year defensive coordinator seemed particularly pleased prior to Thursday’s practice, discussing a group he thinks looks good on paper and has improved in several areas since last season. And already, veteran Mathias Kiwanuka has seen how that confidence has changed his coordinator, which may change everything Fewell can do with his defense this season. “I think he’s more comfortable now. He seems more relaxed and confident in making the calls that he wants to make instead of changing things up here and there for individuals,” Kiwanuka told The Post. “I think he’s got the confidence to make the calls and live with it and trust the players on the field. “It doesn’t matter how much you love to blitz or love a scheme, if you can’t execute it effectively, you can’t put it out on the field because it’s not gonna work. I see the difference in the way he’s approaching it in terms of how he’s confident enough in the people that are out there to make almost all the calls that he has.” In a vastly different looking defense, the biggest changes came up front, with the departures of Justin Tuck and Linval Joseph, and in the secondary, with the arrivals of Dominique Rodgers-Cromartie and Walter Thurmond (whom Fewell calls the “Quiet Assassin”), but much of Fewell’s increased confidence stems from the linebacking corps.