Speaking at a panel in Cannes hosted by the Economist, Schwan spoke of the need for "true partnership" between clients and agencies.
"The client-supplier relationship is very transactional and doesn’t deliver, usually, great creative work," he said.
"If you want to be a good client, don’t be a client, be a partner. Always work on the relationship, always talk very openly about the business side of life, the brand side of life.
"Agencies are not vending machines, right? You cannot throw in money and hope the creative work comes out. It doesn’t work like this, you cannot work like this. You always have to be very close, and that’s sometimes a challenge."
Schwan stressed the importance of good briefs from clients. He said the first brief he wrote was 10 pages long and "terrible", and urged clients to adopt one-line briefs that can be an "inspirational springboard to the creative process".
He also stressed the importance of agencies "embracing a brand" because then "the ideas will flow", saying: "Make sure you have people working for the brand that are... as passionate and crazy as you are, and just get it."
Schwan said PR was "extremely relevant" and "really important" for a "challenger brand" like Burger King.
He gave the example of Burger King’s campaign in Belgium, where the brand is set to make its debut. The campaign, led by PR agency Buzzman, caused controversy by asking the Belgian people to vote whether they preferred the Burger King or their own monarch Philippe.
"We are a challenger brand [so we] have to do things differently, and we don’t have money," said Schwan. "It was a very well thought out plan by the agency."
He said the element of risk "creates energy" and "keeps you awake". He added that Burger King has a "play budget" for campaigns to try different things. "You need to be prepared to do things out of the blue," Schwan stressed.
Also on the panel, Lucien Boyer, CMO at Vivendi, the integrated media and content company, said there had been a shift in spending at the group that had seen campaigns start with earned media.
"We really look at what we can earn from what we do. This is very straightforward," he stated.
This will be followed by shared media, Boyer said, then owned media via Vivendi’s own platforms, and finally paid media. He said the latter "supports some of those projects and fills some gaps", adding that it is something "we always need".