It is just over 48 hours since the BBC Scottish Symphony Orchestra took the breath away by announcing the appointment of Donald Runnicles as chief conductor of the SSO from 2009.

The ink is barely dry on the contract, and the clay from which the relationship will be moulded is, in Runnicles' words, "still soft". But the acclaimed music director of San Francisco Opera and principal guest conductor of the Atlanta Symphony Orchestra, in our first telephone conversation, wants to get a few things sorted out.

Edinburgh-born Runnicles, one of the world's most sought-after conductors, who works regularly at the Salzburg and Bayreuth Festivals, and is virtually a commuter at the Vienna State Opera and the Met in New York, seems aware of interpretations being placed on his decision to take the SSO job: he is the prodigal son returning to his native turf, or the local boy turned international star coming home to his roots.

"Let's please nail that one on the head," said Runnicles, speaking yesterday from Stockholm where he is coaching the soprano Nina Stemme, who will be his Brunnhilde in San Francisco Opera's forthcoming Ring Cycle. "The decision has absolutely nothing to do with being Scottish or returning to Scotland. And I certainly don't feel like a prodigal son."

Nor, he insisted, is it a strategic move to develop yet another string to his bow, following his decision to step down from his prestigious and lucrative San Francisco post in 2009. "I don't think I've ever really made a decision on strategic grounds, I'm coming to the BBC SSO and in no way is it strategic.

"I've been given the chance to work with these fine musicians, and I'm thrilled to be able to find the time to do that."

But why has he taken the job to succeed Ilan Volkov in 2009? And what on earth is bringing a conductor who works in the world's leading opera houses, and with orchestras of the calibre of the Berlin Philharmonic and the Bavarian Radio Symphony Orchestra, to Glasgow?

The answer, from a dozen angles, is always the same: it's the chemistry that ignited when Sir Brian McMaster teamed Runnicles with the BBC SSO at the 2001 Edinburgh International Festival.

The results were electric. "Predicting the relationship between a conductor and an orchestra is a little like trying to predict the weather. Ultimately, when it happens, there's something spontaneous about it.

"I don't mean to sound immodest when I say that, in a very short space of time, during my first encounter with the SSO, I - we - sensed there was a chemistry there that was really remarkable. And with each subsequent project I've done, whether Mahler symphonies, Strauss, Wagner or Berlioz, I've found the SSO a remarkable orchestra, both in its virtuosity and in its spirit and open-mindedness."

While the SSO has gently and consistently worked over the past six years at cultivating the relationship with Runnicles, who is extremely expensive and of limited availability, the conductor was working on freeing up his time from his demanding San Francisco post, which required his presence for six or seven months each year. He loves the work, and will continue as a guest conductor, but was keen to lever some space into his life, both for other work and for his family - he has three daughters, two of whom are pre-teens.

"Having decided not to stay on in San Francisco has allowed me to dream again, feel less bound to the calendar, and weigh up what's important in my life."

As to exactly what he'll be doing with the SSO, only time and future seasons will tell. Those who know him and have worked with him describe him as "both an orchestra builder and a patient musician who will build on the work done by his predecessors. He will bring a distinctive sound to the SSO".

Runnicles describes his own repertoire as wide-ranging. "Symphonically I try to keep as broad a repertoire as possible; I enjoy an eclectic repertoire in Atlanta and San Francisco, and I don't believe it'll be any different in Glasgow, or Dundee, or Inverness, or wherever we go in Scotland."

He describes himself as "deeply committed" to contemporary music, which is part of the remit of the SSO, and will offer a taster of his programming when he visits them in April to conduct Mahler's Das Lied von der Erde and the Third Symphony of James MacMillan, which Runnicles has performed in Atlanta, and of which he describes himself "a great admirer".

"The SSO enjoys a reputation as a courageous flagship for contemporary music, and I want to build on that. And perhaps, having lived over the pond for a while, I'll bring in other works unfamiliar to British audiences."