And now we know exactly what caused the 7-year-old to make this excited face as he entered the chapel at Windsor Castle:
Brian's father, Ben Mulroney (who is married to Markle's best friend, Jessica) explained his son's memorable expression on the Canadian show "Your Morning" on Tuesday.
"I asked him, and he said he'd never heard a trumpet before," Mulroney said. "I think when he walked in, he saw all the people there, and the flowers, none of that was there during the rehearsal."
Sounds like quite the shock!
Mulroney said all of the kids in the ceremony, including Brian's twin brother John and his 4-year-old daughter, Ivy, who was a bridesmaid, had a great time.
"There had been so much talk about getting it right with the rehearsals and these kids are so young," he said. "There was a lot of worry that this wouldn't get done right, but they're having so much fun with it. And that, to me, was the thing that gave me the most pride was that we didn't put so much pressure on them that this wasn't joyful for them. It was pretty darn joyful."
And all in all, Brian and John did a great job as page boys, waving to royal fans and carrying Markle's gorgeous veil.
John, Brian, Ivy and the rest of the crew of kids also kept their cool while posing for official wedding portraits. The photographer who took the pictures recently revealed that kids in the photos were bribed with candy to look at the camera.
"I could hear the kids started crying in the background and there was some chaos," Alexi Lubomirski, who took the official wedding and engagement photos, told People. "And then I heard this magic word behind me that was Smarties, which is obviously an English candy, much like M&Ms, and they were being bribed with one Smartie here and one Smartie there."
He added: "So as soon as the kids came onto the set, I immediately just shouted out, 'Who likes Smarties?' and then everybody hands up, smiles, even some of the adults I think put their hands up."
The sweet deal certainly led to great photos:
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