Once poodles, now alsatians, then one day rottweilers?

The highlight of Bob Stack's career as a non-executive director was when he joined J Sainsbury in 2004 to assist Justin King, the then new chief executive of the British supermarket chain, in the company's dramatic turnround.

"He [Mr King] had to build a new management team, develop a new strategy to drive sales, grow the respect of the brand - all with a new board," recounts the 59-year-old human resources specialist. "To see that take place was very gratifying; to be part of pulling an organisation back from the brink."

Mr Stack has certainly had his fair share of executive experience. As HR director at Cadbury Schweppes in the US - he was born and raised in Pennsylvania - he was bought on to the confectioner's board in 1996. "I've worked for four different chairman over 12 years, and been involved in the selection and succession of the Cadbury board and seen a number of executives move through."

He says it was this process combined with his expertise in HR management that steered him towards his first non-executive role at Sainsbury.

"I knew early on that I wanted to take on a non-executive role; you're dealing with the heart of the organisation. You have a critical role in its most important strategy: the succession for senior management. I don't think there's any more important role than the succession of chief executives, because getting it right or wrong differentiates great companies from poor companies."

With 30 years of HR experience on his CV, he is well qualified for such a role. In that time he has worked for three big multinational companies - Bristol-Myers, America Can (later Primerica and then part of Citibank), before deciding to leave the financial services industry and embark on a career in the consumer sector at Cadbury.

"Every member of a board should have a diverse experience to bring to bear. My background means I have a particular skill in HR," he explains.

"I have the ability to observe team dynamics. Boards aren't a typical team - they meet only a few times a year and there's no opportunity to build strong relations - so if there's a way to help train the group and provide constructive feedback then that's my skill set."

He left Cadbury in 2008 - before the Kraft takeover bid began - and in addition to his role at Sainsbury he now sits on the board at IMI, the UK-based engineering group, and Earthwatch, an environmental charity. He is also chairman of the remuneration committee at both Sainsbury and IMI.

But he says he has probably fulfilled his non-executive quota for the time being. "There's a limit to what one can do. If you're going to be engaged from the bottom up then it's extremely difficult to do more than a handful of these roles. You must make the effort to get out there and meet people [in the business] on an informal basis. In the past, directors didn't have the time because they were on multiple boards. But in the current environment that's an impossibility."

Mr Stack acknowledges that since the financial crisis and the scrutiny that has been directed at bank boards in particular, the role of the non-executive has changed dramatically in terms of risk assessment and accountability.

"Five to 10 years ago people aspired to be a non-executive; it was a no-brainer - the final feather in the cap," he says. "But now you have to be seriously critical in deciding if it's really something that you want to do. It's a lot of extra work carrying potential risk to your reputation if something goes wrong with the business.

"It is extremely challenging to do the job and do it well. The pressure that is out there from shareholder groups and government bodies and the public and the media will actually turn a lot of people off."

He is a strong advocate of "getting the right people", convinced that the days of celebrity directors are over and that the most important criteria are time and commitment they are prepared to put into the role.

"My first experience 15 years ago was that board members weren't as prepared, attendance wasn't good."

He agrees in part with the recent comment made by Roger Carr, chairman of Cadbury, about how non-executives are now the boardroom's alsatians (as compared with the poodle of yesteryear) and that it will only be a matter of time before they turn into rottweilers.

"It doesn't have to be adversarial or aggressive all of the time, but you can't be complacent. You must be vigilant."

And here lies the greatest challenge, says Mr Stack - namely the relationship between non-executives and executives.

"How do you provide guidance without being involved in the day-to-day operations? It's like being a parent," he says. "You're there to provide direction and balance to the children, but you can't get too involved. You have to let the executive team deliver on strategy."