Sometimes, if he talks too quickly or thinks too slowly, he'll wonder why Fred hasn't interrupted him, and then his thoughts'll have caught up with his mouth, or his mouth will have slowed down enough to remember his thoughts. Or sometimes, he'll start a punch line, and wonder why Fred hasn't finished it. Or he'll think about how the Burrow seems so much bigger now, and he'll start to mention it, but then he'll remember the gaping hole in their lives. He'll walk past a mirror and back pass it again, just to check and see if Fred's back from the dead. And when Harry's recounting his adventures and finding the Mirror of Erised, he'll excuse himself to look in the bathroom mirror for a few minutes and wonder if he'd see anything different. Or he'll run into some old friend and they'll accidentally call him Fred and he'll have to pause for a few minutes and wonder where the punch line is. Or his mum will accidentally call for Fred and George before realizing that it's not Fred and George now, it's just George. But inevitably, someone will start to talk about the Battle of Hogwarts, and everything will go deathly quiet, and when Harry gets to the part about Fred's death, George will leave the room and try not to let people see his face. Or he'll have to go to the Ministry for permits and he'll try not to think about the room with the veils. Sometimes, he thinks he can hear Fred whispering conspiratorially in his ear, but then he'll realize that he has neither.
Sometimes, George Weasley will start to crack.