McCullum and captain Ben Stokes have championed England's results-oriented approach built primarily on their take-no-prisoners batting style.
While the aggressive brand of cricket won them 11 of the first 12 matches under the leadership pairing, its effectiveness has since waned after losing Test series in India and Pakistan last year.
In March, England Director of Cricket Rob Key said players needed to "stop talking rubbish" after some made casual comments about not caring about outcomes.
"It's not just about what you do on the cricket field," McCullum said ahead of their one-off Test against Zimbabwe at Nottingham beginning on Thursday.
"It's how you carry yourself. It's how you interact with the public. It's the messaging that you give."
The New Zealander added he wanted the players to "show some humility and not feel out of touch with the general population".
With a home series against India and trip to Australia scheduled later this year, McCullum wanted his team to give fans the same sense of excitement as they did early in his tenure.
"I think people were excited with how we played at the start, enthralled by the free-wheeling cricket we played," he said. "I felt they had a sense of belonging to that type of group.
"But (since then) we have let opportunities slip and perhaps not been as smart as we possibly could with some of our comments to the media."
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