Editor's note: This is adapted from a longer article that appeared on the Bain & Co. website.

Some banking experiences – such as credit card fraud or discovering new rewards available for credit card purchases – have the potential to anger or delight consumers. But when it comes to earning consumer loyalty and even advocacy, everyday interactions actually matter more. For those interactions, the imperative is to make it easy – user-friendly and without hassle.

Making it easy often means making it mobile. But enhancing the customer experience with ease of use in mind is hard for many banks. Whereas a nonbank payments startup may specialize entirely in its mobile app, a bank manages a whole host of different types of interactions with consumers, often customers who hold multiple products. A manager must select only certain types of interactions to determine where to invest in enhancements. So how does an institution set priorities?

Consumers' perceptions can point the way. Bain & Co. recently surveyed 14,800 retail bank customers in the U.S., Australia, Canada and the U.K. We asked them to assign a "Net Promoter Score" (a measure of loyalty on a scale of 0 to 10) to their recent experiences with a dozen common banking interactions, from everyday transactions such as checking a balance or making a deposit to more complex sales and service journeys. The responses shed light on how banks can determine which interactions affect loyalty among their customers and which merit immediate investment.

We found that everyday interactions contribute more to a bank's overall Net Promoter Score than do sales and service interactions. Here's why.

Of any single type of interaction, sales interactions can have the greatest influence on a bank's loyalty score. For example, among U.S. customers, 74% who had had good experiences taking out new mortgages and 80% when opening new checking accounts said they were more likely to recommend their banks to others. On different everyday transactions, the score ranged from 51% to 57%.

However, sales and service interactions occur much less frequently than routine interactions. Factoring in frequency of interaction type, everyday interactions have a greater cumulative effect, contributing three to four times more than sales or service in lifting in a bank's loyalty score across the four countries in our survey.

Our research provides several implications for banks.

The battle is won or lost over routine interactions.

Banks that lead their competition or rank a close second or third on a type of interaction should focus on delighting customers by innovating on their already solid processes. For instance, they could improve their mobile apps to be as close as possible to "one-click," allowing customers to check their balances without needing to log in.

By contrast, banks with low relative scores and a large base of unhappy customers in their everyday transactions should start by eliminating defects in those processes. For example, a log-in sequence that requires the user to scroll down to a new screen on a smartphone should be changed to a one-screen version.

Make it easy through mobile-first design.

Everyday banking is still held hostage to traditional branches and contact centers. At most banks, we estimate that 60% to 70% of teller transactions are bad (because of errors or rework) or avoidable (because they would be quicker, easier and cheaper through digital channels). Such transactions should be migrated to digital—mostly mobile—channels.

Simply installing a functional mobile app or fixing what is broken in the current app is not sufficient to delight customers. An app must be reliable, convenient and easy to use if it is to shift volume away from branches. Now that most customers have used excellent apps from companies in other industries, they expect equally strong mobile features from their banks.

Help your less tech literate customers adapt.

Getting customers to migrate from branch to digital banking hinges on banks having thoughtful processes for changing the behaviors of customers who are technology laggards. In the branch, for instance, employees can ask customers whether they would like to skip the line, then pull them aside to help them set up online accounts and to show them how to make transactions. The second or third visit might require further guidance, until the customer is comfortable using the technology.

Raise the bar on sales and service interactions.

Infrequent though they might be, sales and service interactions have an outsized impact on loyalty. Here too, customers increasingly expect a seamless experience, whether it's digital, in person or on the phone.

As yet, digital sales and service transactions do not consistently delight more than digital routine transactions, but that situation will likely improve over time as banks streamline their digital processes and build greater mobile sales and service capabilities.

Improve customers' end-to-end banking journey.

Customers form their perceptions based on the whole sequence of interactions that they experience. When a hacker makes a fraudulent credit card purchase, a series of interactions follows: alerting the customer, verifying the customer's identity, discussing the process of reversing the charge, counseling the customer on avoiding future fraud and issuing a new card. Many of these interactions span a number of channels and even departments. The degree to which the bank manages all those stages speedily, conveniently and empathetically shapes the customer's perception of the bank.

At a minimum, banks need to measure the consumer experience from end to end. Moreover, delivery of a great experience requires closer collaboration among many departments in executing the operation across several channels and in acting on customer feedback to improve operations. Banks whose departments have historically operated "vertically" need to learn to run "horizontal" efforts that cut across the organization, just as customer journeys do.

To make progress, banks should adopt "agile development," which is well-suited to improving the entire end-to-end customer journey. Rather than building software in one long, "waterfall" process, in which one group of functional specialists hands off a completed phase to the next stage, agile assigns one cross-functional bank team (product owners, marketers, technologists, designers and compliance officers) to go the whole distance as a unit. Agile works best when the problem to be solved is complex; solutions are initially unknown, and product requirements will most likely change; rapid feedback from customers is feasible; and creative teams will typically outperform command-and-control groups.

Banks that master both everyday and emotional customer interactions with a mobile first approach stand to delight more customers and improve their economics as a result.

Mark Schofield, Gerard du Toit and Maureen Burns are partners in Bain & Co.'s financial services practice. They can be reached at mark.schofield@bain.com, gerard.dutoit@bain.com and maureen.burns@bain.com.