If it's the truth, no. But to avoid a defamation lawsuit in the first place, change as many details as you can. For example, it does not fool anyone if the fictional character matches the real person in every respect except that his name is "Jones" instead of "Smith." That could still harm Smith's reputation and he may have grounds to sue. Use your creative license and change the hair, lifestyle, even the gender of the character. Many authors also combine characteristics (making a "composite character"), which often serves dramatic ends as well. You may protect yourself further by placing the familiar disclaimer at the beginning of your manuscript: "The people and events have been changed to protect the innocent, and any similarities to actual persons, either living or dead, are merely coincidental." What also may help is that in most jurisdictions you cannot defame the dead. A sufficiently historical piece may allow you more freedom to write the scintillating details. Of course, if you are writing a nonfiction expose, you have the opposite burden--to make every statement truthful and accurate.
It may not hurt to consult with an experienced libel and slander attorney if you are not sure about the relative risks involved in your literary undertaking.