Fletcher, who has just recovered from injury, felt that laughter and jokes should have been encouraged more among teammates.
Instead, when he returned from injury he found a strange and intimidating feeling around the club.
"When I came back into the first team after being injured, it was a shock to see how intense everything was," Fletcher told The Guardian.
"Not the training - it was hard but anyone can do hard training - it was the intensity of the atmosphere. It was just weird. You had a little laugh and you had the feeling the head teacher was going to tell you off. It was intimidating.
"He [Di Canio] can't deny that he didn't want us to smile and laugh in training but at every other club I've been at laughter's been a big part of things, of creating togetherness."
Fletcher could feature as Sunderland take on Newcastle United this afternoon.