When “Diary of a Wimpy Kid: The Long Haul” star Alicia Silverstone waved the green flag to start today’s INDYCAR Grand Prix, she already had first-hand knowledge about the skills of the drivers she watched speed by under the Indianapolis Motor Speedway flag stand.

A few hours before, Silverstone, who plays the matriarch of the Heffley family in the most recent installment of the “Diary of a Wimpy Kid” film series, and co-star Tom Everett Scott took thrill rides on the road course in the INDYCAR Experience custom two-seater. Silverstone rode with racing legend Mario Andretti.

“I’m shaking a bit and I screamed the whole way,” Silverstone said after she got out of the passenger seat. After the experience, Silverstone asked herself why racers love their jobs so much.

“Why would you want to do that? It’s really intense,” she said. “But what skill. When he stopped – whoa! – you think you’re crashing! I kind of can’t process it. It’s daredevil. It takes massive skill. But, really, as a person sitting in the car, it’s intense and I’m nauseous.”

Silverstone, Scott, 11-year-old Jason Drucker, who plays Greg Heffley (the main character in the “Wimpy Kid” series), film director David Bowers and “Diary of a Wimpy Kid: The Long Haul” author Jeff Kinney were all in Indianapolis promoting the May 19 release of the movie, which centers around young Greg Heffley’s plot for his family to take a cross-country trip to Indiana for his Meemaw’s 90th birthday and, at the same time, attend a nearby gaming convention. The cast members spent part of race morning meeting fans and signing autographs.

Also among the celebrity contingent at the INDYCAR Grand Prix on Saturday was retired late-night talk-show icon David Letterman, who’s also the Rahal Letterman Lanigan team co-owner.

From the NFL, Indianapolis Colts quarterback Andrew Luck and his girlfriend, Nicole Pechanec, rode bikes to Indianapolis Motor Speedway before the race before Luck gave the “drivers, start your engines” command. Newly retired Colts outside linebacker and the team’s all-time sack leader Robert Mathis drove the celebrity pace car to start the race as Silverstone waved the green flag.

Hollywood Records boy band Forever in Your Mind (shown above right with INDYCAR driver Gabby Chaves) sang the national anthem and spent time with IndyCar driver Jack Harvey, while touring his garage. Band member Liam Attridge got a two-seater ride, too.

Meanwhile, Scott and Drucker took in the race from the sidelines. They toured the Andretti Autosport garages before the race and met 2014 Indianapolis 500 winner Ryan Hunter-Reay.

Scott adapted to the two-seat Indy car ride better than Silverstone.

“I loved every second of that,” he said. “The ability to hold on to those cars, it’s a special skill. Those drivers are performing at a high level. It’s fun just to get in the (two-seat) car, and they aren’t going at top speed. So, in a competitive environment, I can’t even imagine.”