Celebrity arrests, affairs and social media snafus fill infinite tabloid pages and fuel a myriad of gossip sites, making it seem as though that's just life today as a Hollywood star. But what happens when that life touches Madison Avenue?

Brands with celebrity spokespeople face unique challenges from finding the right fit for a brand, vetting them properly and then managing them through the shoot and beyond into the real world. It can be a journey of both angst and opportunity. So we tapped some agency veterans whose job it is to deal with celebs and asked: how do you do it?

Start with the creative idea
"Everything here starts from the creative. The best celebrity campaigns start from the idea and then add the celebrity dimension to it," said Peggy Walter, VP-director of celebrity services at Leo Burnett. Ms. Walter began as a talent agent, but has been in the Burnett role for 20 years and before that help a similar post at DDB.

When the strategy is laid out before a celebrity is even considered, it's much easier to continually refer back to the plan built around what the client needs. "It sounds boring, but it always goes back to the creative," she said.

Choosing actor Dean Winters as Mayhem for Allstate is one example. The idea was to use the opposite of what Allstate stands for with an actor who added "a little streak of crazy or a bit of danger," said Ms. Walter. Mr. Winters was then acting in "30 Rock" as Liz Lemon's no-good ex, had played bad guys on "Oz " and "Rescue Me" and had enough renown, as Ms. Walter said, to communicate the anti-safety message the marketer wanted to get across. She added that his acting ability helps: "He's also a really good actor which we knew we wanted in that role."

Make sure celebrities are additive to the brand message
Nick Phelps, the Droga5 global alliance director who handles the agency's relationship with William Morris Endeavor said, "You see a lot of commercials where the celebrities come first where people remember the celebrity, but don't remember what it was for. Celebrities must help deliver the message. It's easy to fall into the trap of the celebrity as the idea."

Mr. Phelps said ballet dancer Misty Copeland and supermodel Giselle Bundchen for Under Armour weren't the expected choices for a mass sports brand campaign, but their determination and will in the face of immense criticism enhances the message "I Will What I Want."

Vet celebrities carefully and include face-to-face interviews if possible
Simply asking them "Why do you want to do this?" can be an effective predictor of potential accord, said Leeann Leahy, president of the Via Agency. That conversation should also include the agency taking a turn explaining why it thinks the celebrity is the right fit.

The benefit of an early buy-in can lead to better collaboration and co-creation of a property. For the Budweiser "Made in America Festival," that's what happened with musical artist Jay-Z, she said. Building the event together gave Jay-Z more ownership and a bigger interest in its success.

Make requests upfront and be as specific as possible about expectations in the written contract, such as number of shooting days needed and social-media expectations.

Evaluate missteps by a celebrity spokesperson on a case-by-case basis, and act quickly
People make mistakes, and while celebrities probably make them in the same proportion as "normal" people, the media will be on hand to record their screwups. Take into account, however, that there are different kinds of errors. Brand A's celebrity spokesperson getting drunk at a party and tweeting a goofy comment may be one thing; Donald Trump advocating harshly against immigration is another.

"There are mistakes that at a fundamental value level are different than what you bought into," Ms. Leahy said. "If the behavior is inconsistent with the brand values, then it is the responsibility of the brand to walk away."

That said, it's important to act quickly, Mr. Phelps said. Standing by the celeb is fine, but make your case as soon as possible. If you're cutting ties, also explain why and soon.