Fleming, who stayed at home during the recent Pakistan series, is in England for a fortnight meeting cricket board officials, Test skipper Joe Root and key figures in county cricket .
He is due to link up with McCullum's limited-overs squad in Manchester ahead of the third T20 against Sri Lanka on Saturday.
McCullum made it clear the tightening of disciplinary expectations that Root has already made a priority will be mirrored in his regime.
Brydon Carse has been left out across all formats since being arrested, then de-arrested, outside a Derby nightclub last month – treatment that speaks of a stricter philosophy than the freewheeling early days of 'Bazball'.
"I hope it's crystal clear what is expected, not just of this group, but of those who have got aspirations to play for us," he said ahead of the series opener at Southampton.
"We want to play a style of cricket that comes with responsibilities. That's just life living in the fast lane of international cricket. There is a lot of talent around, if you're not prepared to do that, there are other guys who will be given those opportunities.
McCullum's message was reiterated by his high-profile new arrival, Kevin Pietersen. The former England captain has joined the white-ball backroom team in a mentoring role.
He has previously been critical of McCullum's outlook, admonishing the squad for a lack of preparation in India last year as well as raising questions about their approach to the Ashes.
"When I rang him and said 'would you be interested in working with England?' I think he thought I was joking. He mentioned, 'I've been quite critical of you guys in the past' and I said 'I have no problem with that, mate'. There's parts of it I agree with and parts which I won't agree with and that's fine. That's the beauty of Kev coming in.
"I want him to look across the environment and challenge me privately around ways we can improve or things we can do differently.
England will give a debut to big-hitting Aneurin Donald at Southampton. The 29-year-old will partner Jos Buttler at the top of the order as a reward for some blistering form in domestic cricket.
England's top six will also include captain Harry Brook, Tom Banton, Jordan Cox and Will Jacks.
Jamie Overton, Sonny Baker and Josh Tongue will serve as the seam attack, with Jofra Archer rested, while Liam Dawson and Adil Rashid are inked in as the first-choice spinners.
