"I didn't expect to come into the team straight away but unfortunately for John he's been out with a long injury," Bunn told BBC Radio Norfolk.
"He's due back soon so I've got to be on top of my game to keep him out."
The former Blackburn Rovers stopper has been first choice between the sticks, featuring in 16 consecutive league games and keeping four clean sheets in his last eight appearances.
"I love playing. It's been a while since I've played quite a few games," he said.
"From a goalkeeper's point of view it's always good to keep clean sheets. That's why I'm there, to make saves.
"Hopefully I can keep another clean sheet in the next game [against Sunderland]."