Brie Larson had the best reaction to getting caught in the middle of a fan's proposal

Brie Larson got a big surprise when she found herself in the middle of a proposal at ACE Comic Con in Chicago over the weekend and had a priceless reaction to being part of the big occasion.

The Captain Marvel actor was posing for pictures with two fans at a meet and greet when one of them, John Chambrone, got down on one knee to pop the question to his partner, Richard Owens.

The amusing moment was captured on camera, with one photo showing a shocked Larson looking toward the camera as Chambrone grins with his arm round her waist while a visibly stunned Owens holds his hands to his mouth.

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Larson looks between the happy pair in subsequent images before the final shot shows her peeping out from behind the kissing couple, still wide-eyed.

She expressed her delight at being witness to the momentous event on Twitter.

"Y’all I was present for a proposal and I am CHANGED. Much love to the happy couple!!!!" Larson exclaimed as she shared the photos of the moment posted by another fan.

Chambrone later shared his excitement at the encounter in a post to Facebook as he described it as "the most epic moment of my life".

Meanwhile, last week saw Larson reveal that the female members of the Marvel Cinematic Universe approached studio boss Kevin Feige to make an all-female superhero movie.

She explained it was something they "are really passionate about" and that it “maybe” could happen if the demand from fans was there.