Until a few years ago, when companies needed a quick and cheap way to make a splash, they would simply use a star's face and name in an endorsement message without permission.
No one dares be that blatant now, but that is not stopping clever marketeers from probing the limits of what is allowed.
These days, they place advertisements featuring a famous person, congratulating him or her for an achievement. A recent example of this happened after Singapore swimmer Joseph Schooling won Singapore's first Olympic gold last year.
The win triggered a flurry of congratulatory advertisements from fast-food chains and transport companies, among others. Each one featured a corporate logo, sometimes with a discount to mark the historic occasion.
Associate Professor David Tan from the National University of Singapore Faculty of Law (NUS Law) says companies that run such advertisements might be said to be "stealing" value, even if they do not mean to do so.
"They are suggesting that the star endorses the product... they appear to be congratulating, but are, in fact, avoiding paying the licence fee," says Prof Tan, who is vice-dean of academic affairs at NUS Law and conducts research on copyright, trademarks, personality rights and freedom of expression.
In the United States, a grocery chain bought such an advertisement in 2009. It showed its brand and the name of basketball legend Michael Jordan. Jordan sued and a higher court ruled that the graphics and text exploited Jordan's fame for commercial purposes.
"They were siphoning his halo effect," he says, referring to how stars have a halo of status and glamour around them that can be passed on to things they endorse.
Prof Tan, 47, spoke to The Straits Times following the launch of his legal book, The Commercial Appropriation Of Fame.
The laws around fame have also had to adapt to the age of social media, with people who are famous for having thousands of followers on YouTube, Instagram or Facebook, he says.
In the past, celebrities needed to possess a talent - for acting, dancing or some other skill, he says. These days, the definition of fame has become somewhat circular.
"I define a celebrity as someone who is well-known for his or her 'well-known-ness'," he says.
At the book launch last week, held at the NUS' Bukit Timah campus, singer-actress Kit Chan and television host Allan Wu, both friends of Prof Tan's, spoke about the pressure to create a social- media image that attracts followers.
Wu, 45, says he has noticed a pattern with celebrity pals who post images of lavish restaurant spreads.
"I know full well they will not consume that food... They eat like rabbits. That's how you play the game, " says the host of The Amazing Race Asia and The Amazing Race China.
Wu, who has 28,100 Instagram followers, says that in his posts, he has trained himself to overcome his instinct to shield himself and his children from public view.
"I post my kids, topless shots, travel shots, I post pretty much everything," says Wu, who has two children with his ex-wife, former actress Wong Li-Lin, who has 35,800 Instagram followers.
Kit Chan, 44, says she prefers not to reveal too much and most of her social-media posts are work-related, in contrast to Wu's more open attitude.
"I keep friends and family away from publicity. It's something I'm strict about. They did not choose to be part of what I do," she says.
•The Commercial Appropriation Of Fame is available at Books Kinokuniya for $163. The e-book is available at Cambridge University Press (tinyurl.com/ycyuxdz5) for US$88 (S$119)