To hear senior marketer John Fleming tell it, Wal-Mart Stores’ marketing mission is pretty similar to that of other retailers: to improve the in-store connection with customers. The level of complexity, though, increases dramatically for a company with roughly $350 billion in sales and 6,000 stores around the world.

Fleming has a unique perspective on connecting with Wal-Mart’s customers, thanks to his past roles as president and CEO of Walmart.com and as Wal-Mart’s chief marketing officer and to his current one as the company’s chief merchandising officer. In this interview with McKinsey’s David Court, Fleming explains how marketers at retailers can make the greatest impact: “Think like marketers,” who use data to make decisions, “but act like merchants,” who react rapidly to market changes.

Like many companies, Wal-Mart grapples with the fragmentation of customer needs. A key implication, says Fleming, is that it must segment its customers more finely than it has in the past and develop a variety of different in-store experiences. That creates execution challenges and makes managing Wal-Mart’s brand more complex.

Fleming also discusses the current role of Walmart.com, which is not only a distribution channel but also a powerful marketing vehicle. And he describes the importance...