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"Man is only truly great, when he acts from his passions,” said the indisputably great former British Prime Minister Benjamin Disraeli.

For the past 10 months, since replacing Larry King as host of CNN’s 6 PM interview show, I’ve sat across the desk from some of the most famous, powerful and fascinating men in the world. I’ve seen all manner of human emotion from them: tears, anger, heartache, arrogance, humility, hilarity. But the key component in all these successful men—whether they lead the field in politics, entertainment or business—is passion. And this can be a dangerous, as well as an exciting virtue.

Charlie Sheen: Never Dull 
It was another famous Benjamin—Franklin—who cautioned, “If passion drives you, let reason hold the reins.” Which brings me neatly to Charlie Sheen. I’ll declare an interest here. I like [Sheen]. I’ve always liked him, ever since I interviewed him in Aspen in the early ’90s, and he made me laugh like a hyperactive chimpanzee with his razor-smart wit and infectious charm. His appearance on my show was one of the most unpredictable, dramatic and popular so far. He turned up five minutes before we went live. We’d “lost” him for two nerve-wracking hours. He arrived falling out of a Maybach with the weirdest looking entourage imaginable, a cigarette hanging out of his mouth and uttering the words, “OK, Piers, let’s go rock ‘n’ roll.”

Sheen was a compelling guest: raw, visceral and confrontational, yet warm, funny and surprisingly profound, too. His power came from his passion. Say what you like about Mr. Sheen, but he inspires extreme opinion wherever he goes—he’s never dull. “I believe fundamentally in a man’s right to live his life how he chooses,” he told me. It’s an ideology that surely carries merit so long as others don’t get hurt in the process.

My godfather, a successful UK businessman, once told me the trick in life is to get “F*** You” money as fast as possible—enough cash in the bank that allows you never to make decisions based on financial necessity. Sheen personified that ethos when he walked away from the highest-paid gig in world television.

Whether you approve or not of his personal lifestyle, it’s hard not to be impressed by the willpower and independence of a man prepared to give up a job he loved, at the peak of his earning power, on a point of passionate principle. I’m not saying Sheen’s perfect, because that would be absurd. He’s a deeply flawed character in many ways. But he’s extremely amusing and considerably more self-aware than people assume. During a commercial break toward the end of our interview, he handed me the negative drug test report he had proudly brandished earlier to the viewers. On it, he’d scrawled, “To Piers, let’s get hammered, love Charlie.”

Nelson Mandela: Greatness
Passion comes in many guises. Take Nelson Mandela, the most impressive human being I’ve ever met. I had a private audience with him in London in 2003 after he gave a blistering speech, without notes, against the Iraq war, ripping Bush and Blair to verbal shards. The furious passion blazed from every pore, as did flashes of belligerence, obstinacy and single-mindedness. Beneath that beaming grin, he’s a tough, hard man.

But the most striking thing about Mandela, as anyone who encounters him can testify, is the total lack of bitterness from someone who spent 27 years in prison. He came out to cry peace and forgiveness, not war. And as a result, South Africa has been transformed into a genuine, multicultural, racially tolerant nation. And he has found happiness and peace of mind from knowing he did the right thing.