Kristen Johnston has never been afraid to be smart, funny, and honest -- whether on stage in The Women or So Help Me God!, on the small screen in the hit series Third Rock from the Sun and her current show, The Exes, or in her new book Guts, due out March 13 from Simon & Schuster, in which she candidly deals with her years-long struggle with substance abuse and addiction.

"I am amazed I finished it," says Johnston. "And I wrote every fucking word. It took me a year-and-a-half and it was just so difficult. It was way harder than acting. I learned how to write within the process of writing. There were times, though, I realized I just had a great love of creating the perfect sentence. But the most important thing about the book is I wanted it to sound like me. And I wanted people to know that I'm a real person -- not some preconceived notion of a Hollywood actress who throws things at people. One of the reasons I wanted my first chapter to break the fourth wall, as we say in theater, is so that people felt like I was talking to them."

Johnston, who calls herself an avid lover of reading, originally had a different idea for her first book. "I was going to start with sort of a funny self-help book for young girls and I brought that idea to Simon & Schuster, and, of course they wanted writing samples," she recalls. "So I offered to give this editor some of my papers from my time at NYU, and she said no. So I sent her 12 emails I wrote, and when I went to get the longest one I could find, it was this one I wrote to my friend Chris when he got sober and I was five months sober. And my editor wrote me back and she told me, 'let's do that book.'"

While she hadn't really thought her life was the right subject for a book, she soon changed her mind. "At first, I thought I could never write about my addiction - I'm not one of those people - and then about 10 minutes later, I just started writing about it," she says. "It's not a mea culpa book, and I'm not interested in having a pity party. And it's not a typical celebrity memoir. It's a memoir of someone who started life over. I've tried to tell my story, and though it's a dark story, I try to tell it with humor and to make it an entertaining story."

Johnston believes the book is vital reading not just for addicts, but for people who are involved with addicts, whether they realize it or not. "By writing this book, I am choosing to make the subject of addiction less embarrassing. I think secrecy and shame is what is killing us," she says. "People don't realize that alcohol-related or drug-related deaths are the biggest killer of people in this country, and that's partially because people don't talk about it. One out of three teens in the U.S. meets the criteria for addiction. So it's not just Whitney Houston or some narcissistic Hollywood actor who's an addict; it's your mom or your cousin. And if you don't understand what addiction means, you'll never know why an addict chooses the door the drug is behind, and not the door the family is behind."

Having never done any professional writing before, Johnston turned to other memoirs for inspiration. "You know, James Frey's A Million Little Pieces is really a great book. Even if it's bullshit, it's the closest anyone has come to describing the pathos of the struggle of addiction," she says. "But the person who most inspired me was David Sedaris, because in each of his books, his chapters have a beginning, middle, and an end. I really tried to take any chapter of my book out of context and see if it still makes sense."

She showed parts of the book along the way to friends, including director Joe Mantello, the actor and writer David Rakoff, and Oscar-winning actress Kate Winslet -- all of whom she calls incredibly supportive. But one person has yet to see book. "I'm most nervous about my mom's reaction," admits Johnston. "She's a private woman and I know this will be difficult for her -- especially how some people will feel it reflects on her. But it's not meant to be accusatory. And when I told her I was writing it, she said, 'I don't think you should write it, but it's your journey.'"

The book is just one part of Johnston's crusade to get America to deal with the perils of addiction. Five years ago, she founded SLAM, whose goal is to get a "sober high school" built in New York City. Last month, she organized a special evening of the long-running show Celebrity Autobiography in New York as a benefit for the organization.

"In this country, one in 70 kids go to rehab every year, and if they go back to a regular high school, their chance of relapse is 90 percent," she says. "But if they go to a sober high school, they are much more likely to stay away from drugs or alcohol. (For more information, visit www.slamnyc.org.)

While she only has a couple of months before she returns to Los Angeles to film the second season of The Exes, she will be doing some readings and book signings, and encourages people to communicate with her via her website (www.gutsthebook.com).

And as the book documents, her life hasn't always been easy or perfect, but she has no regrets. "I wouldn't change anything that's happened in my life, because everything has led me to the life I have right now," she notes. "I am not just happy, but really proud of who I am right now."