Facts in Fiction
Today one of my interns asked this question:
Can a character in a novel eat at McDonald's in the booth next to Brad Pitt while listening to her favorite group, U2, on her iPod after just having seen James Cameron's Titanic? I'm thinking that the answer is yes with stuff and places, but for some reason people seem to be a bit trickier.
Generally, McDonald's, U2, iPod and movie-makers love the free advertisement. Sometimes they even pay novelists or script writers for "product placement." Think of Ray-Ban paying celebrities to be seen wearing their sunglasses, and paying them even more when somebody runs a photo of said celebrity in Ray-Ban glasses. So from a product-maker's POV, you're fine. In False Positive we included places like Starbucks, restaurants such as Dallas's Star Canyon, and products like Chanel No. 5.
But you're right--it gets a bit trickier with people. Yes, Forrest Gump meets JFK, and we know JFK never met Forrest, because Forrest is somebody's made-up creation. So if it's obvious enough to make readers smile, knowing it never could've happened, I think you're okay. Seventy years ago Upton Sinclair created his eleven historical-fiction novels, the Lanny Budd series, in which the illegitimate son of a tycoon keeps showing up in the center of history’s momentous events. In the process he interacts with Mussolini, Hitler, Göring, Goebbels, Roosevelt, and Woodrow Wilson. In historical fiction you can do this, but you're generally constrained by the actual events.
Herman Wouk's WWII novels, The Winds of War and War and Remembrance, included numerous fictional people interacting with political figures who actually lived. Historical fiction is built on such interactions.
The main concern with real people, especially if they are living, is over slander/libel or hacking them off and getting yourself sued. The publisher usually decides how much risk they want to take here. Having a living person do something he or she didn't really do might raise legal questions that a publisher wouldn't want to face. Still, I'd write the story how you want it, and as long as you don't make a person look bad or slander him or her, I think you're probably okay. The general rule of thumb comes back to "do unto others..." An advertiser wants people to talk about the product. But a private citizen may or may not appreciate appearing in somebody's story--depending on how he or she gets portrayed.
If I were writing the scenario you described, I'd try to find out what restaurants Brad Pitt actually frequents so that even if he himself read the work, he would say, "That's possible..." instead of "That's crazy! I would never eat there. I have way more class than that..." And I would limit what Brad does in the scene to something in character for him. I would not have him get in a fist fight. Now if the celebrity in the restaurant were Russell Crowe...