Now, he’s being put in charge of our whole team, and I can foresee real trouble ahead if he doesn’t change his behavior. He has invited us to give him feedback, and I’m dying to, but the speeches I’ve rehearsed in my head just sound like complaining. What’s the best way to tell someone they need to get their act together without wrecking the relationship, or coming across as a whiner? — Tongue-Tied

Dear T.: Funny you should ask. It seems that most of us have something we’re trying very hard not to tell someone at work. David Maxfield and Joseph Grenny, co-authors of a book called Crucial Conversations, recently did a study of 1,400 U.S. employees and managers and found that 56% refrained from bringing up a particular grievance, or more than one, usually out of fear of alienating someone they have to see every day.

What’s more, most of them reported keeping mum for more than a year. “I was amazed at how long people kept things bottled up, usually until they were ready to explode,” says Maxfield, who is vice president of research at consulting firm VitalSmarts.

That reluctance to speak up is especially surprising, he says, “when you consider that most people admitted that an honest discussion would do the other person’s career, and the organization’s performance, a lot of good.” Two thirds (66%) told researchers they believed expressing their complaints would help their company achieve its goals, while 43% thought the offending coworker would be better off, and 39% said “a huge emotional burden” would be lifted.

Your new team leader’s openness to feedback (assuming he’s sincere about that) is a good thing, because you need to clear the air. “If you don’t talk it out, you’ll act it out,” says Maxfield. “You could start getting grumpy, avoiding the other person, or being abrupt with him, all of which reflect badly on you, as if you’re the one with a ‘bad attitude.’” And who needs that?

Maxfield suggests these four ways of approaching the conversation:

Assume that people can change. More than half of the employees in the survey say they haven’t spoken up because they don’t believe the other person could or would change. “But people do change all the time,” says Maxfield. “Ask yourself, ‘If I were in this colleague’s shoes, and I had a true friend who knew what I know, would I want them to tell me?’ Most of us say, ‘Yes, of course’ because we have confidence that we can change. So do your new team leader the favor of letting him try.”

Figure out what you really want. When people rehearse their side of the discussion (as you’ve been doing), they tend to get hung up on venting their frustrations by spouting “accusations, not aspirations,” Maxfield notes. Vent to your friends, your significant other, or your dog, but before speaking to your colleague, calm down and “ask yourself what you really hope to accomplish — not just for yourself, but for the other person and for the working relationship.”

Make your motives clear. We live in what Maxfield calls “a low-accountability culture,” where any criticism is easily perceived as an attack (perhaps because, all too often, it is one). “Remind your team leader that you are on the same side, and that your intention is to help,” Maxfield suggests. At the outset, state that your goal is for your team to be successful. You don’t need to mention the “real trouble ahead” you say you foresee if he doesn’t shape up. Instead, Maxfield says, focus on how your feedback can help him be more successful — the other, more positive side of the same coin.

Stick to the facts. “Complaints that people have kept bottled up for months or years grow big and hairy,” Maxfield observes. “The specific problems get lost beneath layers of conclusions like, ‘you’re just flaky,’ or ‘you don’t care about the rest of the team.'” Avoid blanket statements like that (which Maxfield says are often inaccurate anyway, or at least reflect only one side of the story) and be ready with specific examples of situations where your coworker “flaked out” on you, as you put it. Says Maxfield, “If someone is in denial and disputes what you’re saying, the facts — including what you expected and what actually happened — will speak for themselves.” Good luck.

Talkback: Have you ever sat a coworker down for a candid discussion that was critical of his or her job performance? How did it go? Leave a comment below.