Berkshire Hathaway (BRK-A, BRK-B) CEO Warren Buffett, the seventh richest person in the world according to the Bloomberg Billionaires Index, will release his widely anticipated annual shareholder letter on Saturday, revealing his thoughts on the state of the company and the outlook for the stock market, among other topics. In a new interview, billionaire Thomas Tull — who runs a holding company called Tulco modeled in part after Buffett's — described just such a piece of advice from Buffett that shifted his approach to investment decisions.