There are few tasks more critical--and more tricky--than bringing on a new manager. After all, the right person can bring new energy and creativity to a company, and free you to concentrate on the grand vision. The wrong one can irritate employees, customers, vendors--in short, do real damage. The difference between success and failure? It almost always depends on what happens during the new manager's first three months on the job--or so says Harvard Business School professor Michael Watkins, who studied the experiences of hundreds of managers who were hired with high expectations, only to fail at their tasks. Watkins, whose findings are collected in the forthcoming book The First 90 Days, recently spoke with Inc. senior editor Mike Hofman.

So much for the honeymoon. Why are the first three months so crucial?

Employees who work for a new manager make judgments based on remarkably little data. So do a manager's peers. And those judgments are sticky. Successfully building credibility early can propel a manager through a lot.

What's the most common mistake business owners make when it comes to new managers?

Simply not paying attention during the assimilation process. You can't just hire a person and say, "Okay, go." A person needs to work with you to learn the company, the culture, the political situation.

Spending a lot of time managing the manager...sounds like micromanaging.

But the impulse to not spend time with new managers is deadly. It's especially deadly for business owners, because they tend to have extremely strong ideas about what works and what doesn't. On an emotional level, business owners are often not prepared for change. If you don't set out your expectations and values early, you are likely to be at each other's throats.

How do you spot when new managers are getting into trouble?

You can always look in their eyes for the hunted look. And obviously, if they're pissing off a lot of people, that's a problem. But you should also look to see whether they are moving out of their comfort zone. Some managers will immerse themselves in functional expertise like marketing or finance as a means of avoiding the big, cross-functional issues they really ought to be grappling with. That's a big warning sign.

How should you intervene?

Get managers into your office and see where they are struggling. Is it style? Is it resources? Ask how things are going without putting them on the defensive. But if they're flailing and they're unwilling to talk about it, it's now a matter of managing their exit.

Can a bad start be reversed?

That depends on how poorly it goes. If new people make major business mistakes and alienate people, they're cooked. If they're merely mediocre, they've still got a shot, but they have to nip vicious dynamics in the bud. What can often happen is that they make a bad call, and the people around them draw back from sharing critical information, which sets them up to make another bad call.

When a new manager fails, what's the cost to the business?

A bad hire can be an inconvenience for a large organization. For a small business that doesn't have the same margin for error, a new hire in the wrong role at the wrong time--that can be a near-death experience. The leverage of an individual in a small business is huge, which is why small-business owners have to move beyond "sink or swim."