Last summer, Marc Andreessen and Ben Horowitz stopped by the Fortune offices to be interviewed for an upcoming story. The pair had just launched a new venture capital firm, but the editorial focus was Andreessen, a Silicon Valley icon since co-founding Netscape at the age of 23. Horowitz, who once had worked several pay grades below Andreessen at Netscape, didn’t even appear in the cover shot.

This was not an isolated incident. In fact, the only unusual part was that Horowitz showed up at all. In most other interviews – including a televised sit-down with Charlie Rose – Andreessen is on his own.

“It was mostly a tactical thing,” Horowitz explains. “We had plenty of work to do setting up the fund, and my feeling was: ‘Marc, if you want to do the press, go for it.’”

In the subsequent 15 months, Andreessen Horowitz has become one of the hottest venture capital firms in the industry. It made national headlines for helping to carve Skype out of eBay (EBAY), and then won a hotly-contested race to invest in Foursquare. Today, the firm announced it’s raised $650 million for its second fund – an unheard of amount for a firm of Andreessen Horowitz’s age and size.

Despite his relatively low profile, Horowitz clearly deserves his half of the credit. His entrepreneurs view him as some sort of operational oracle, identifying potential problems before they arise.

“During dinner after closing our financing, Ben began talking about all sorts of things we hadn’t thought of,” recalls Foursquare CEO Dennis Crowley. “For example, he asked how big our engineering organization was. He then told us that when it reached this certain point, it would break and we’d need a fulltime recruiter for more engineers. Then he said that at a certain point we’d need to add these types of customer service resources. It was impressive.”

Problem solver

Horowitz honed his CEO skills as head of Opsware, a provider of data center automation that nearly collapsed before rebounding and being acquired by HP for $1.6 billion. The experience taught him not to get too low when things get tough, and also that one fix doesn’t always solve the same problem.

“A lot of other VCs pattern recognize, telling their CEOs that something that worked 10 years ago in a different situation will work for us today,” says Steve Mullaney, CEO of Nicira Networks. “But Ben is an original thinker who looks at our issues in context.”

Such aptitude is why Horowitz usually manages investments where the primary challenge lies in execution (Andreessen Horowitz is more “horizontal” than most other VC firms – typically matching partners with portfolio companies at the end of due diligence). He has board or board observer roles at Foursquare, Proferi, Nicira and Skype. If the company is still trying to figure out its product – or “big picture” – then Andreessen often takes charge.

This formula has helped Horowitz feel that he’s staying true to his operational roots, despite not having a chief executive title. It also has helped him feel more comfortable as a venture capitalist, which in Silicon Valley often doubles as a public figure.

Earlier this year, Horowitz began to blog. Not just about his daily activities or playlist, but detailed thoughts on investment and technology issues.

One notable post was on the value of “fat” startups in a lean world, which later was challenged by veteran VC blogger Fred Wilson. The two later debated the issue during a large tech industry conference, which helped push Horowitz even further into the spotlight.

“If someone’s going to sit down and read one of my posts, I want them to get maximum value out of it,” Horowitz says. “I did the fat vs. lean debate because I think it was a high quality discussion in which people who heard both arguments really learned something, even if they thought Fred won.”

Christian Gheorghe, founder and CEO of Proferi, believes that Horowitz has “found a voice” in his blog postings and speaking engagements. “A lot of people probably only knew Ben’s name because of Marc at first, but now a lot of them know him for what he thinks and what he’s doing.”

In fact, there’s even a small chance that Horowitz will do some of the press associated with his firm’s new fund close. “I’d be lying if I said it hasn’t come up,” Horowitz says. “My plan was to tell them that I’d be in New York and unavailable, but it doesn’t look like it’s going to work this time.”