“The Proposal” Costumer Recalls 'Hilarious' Betty White in Wedding Dress Fitting Scene: 'Bless Her' (Exclusive)

15 years after the movie's release, Catherine Marie Thomas tells PEOPLE the custom piece that Sandra Bullock wore was made even better by White's ad-libbing in the film

Some projects — and some people — stay with you long after you leave them behind.

That was the case for costume designer Catherine Marie Thomas, who worked on The Proposal, which was released exactly 15 years ago on June 9, 2009. Thomas tells PEOPLE that it was a "very special group" of people who came together to make that movie.

Starring Ryan Reynolds and Sandra Bullock, the film tells the story of an editor in chief, Margaret (Bullock) who needs a green card, so she enlists her assistant Andrew (Reynolds) to marry her. The two lead characters have wildly different personalities, which Thomas balanced through their wardrobes, but she tells PEOPLE that one of her standout moments from filming came from the late Betty White.

"Betty White can do no wrong," she says of the late actress, who died on Dec. 31, 2021 at 99. "I mean, really and truly, this was such an honor to work with her. She's so funny, so sweet. But the wedding dress scene! It's hilarious. God bless her."

In the scene, Margaret is gifted with Grandma Annie's (White) wedding dress, and she tries it on for Annie to tailor. However, the dress doesn't fit her. At all. (This is a comedy, after all.) Thomas says White ad-libbed parts of that scene as they shot it over and over, making it funnier each time — and it was already funny to begin with.

But at the core of the scene, Thomas still had a beautiful dress — that was actually two dresses.

"We backtracked by thinking that if it was Annie's dress, it would've been probably from the 1930s or somewhere in there," Thomas says. "So we just started looking at stuff from the 1930s with Sandy because she loves to, and it was a really fun process. She loved the idea of having something really elegant but sort of covered."

Thomas continues, "We started by figuring out what it would look like on Sandy, and then we made another version of it in a comically large scale [for the fitting scene]. It's actually harder than you would think to make something look like that but in a bigger scale."

Recalling the scene still makes Thomas smile, she says, because Bullock and White had so much fun with it, especially with White really going for the joke about how "chesty" she was in comparison to Bullock.

"She was beyond a treasure," Thomas says.

Related: Betty White's Life in Photos

The costume designer, who also worked on projects like Kill Bill, Mr. Robot, The Heat and more, says that working on The Proposal was a unique challenge because the story takes place in such a short time period: essentially a weekend. Margaret and Andrew visit his family in Alaska for the weekend, so much of what Thomas had to work with clothing-wise had to theoretically fit into the characters' weekend bags.

"It's tricky when you have a storyline that takes place in such a concise period of time," she says. "You know what I mean? To be able to tell that story without it feeling totally unrealistic. You don't want the viewer to be like, 'Where'd she get that from?'"

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Before the duo left the big city for Alaska, though, viewers meet hardcore Margaret, and Thomas crafted a custom wardrobe for her that took her level of uptight to an extreme.

"That style of suit didn't really exist at the time," she says of the custom pieces she put Bullock in at the beginning of the film. "It was to the point where she almost couldn't walk in that long pencil skirt. It was sort of 1940s-style. There was definitely an homage to that silhouette, plus the big shoulders. Now I think you see it more, that very masculine yet feminine silhouette. It was this play on making her feel stilted but also very powerful and sexy."

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Thomas wanted Bullock's character to slowly loosen up while in Alaska, but there was a short timeframe — and limited luggage space — to do so. Plus, Margaret doesn't truly loosen up in just a few short days.

"She's very fashionable, so she doesn't ever let go of this or the shoes and the Birkin bag. If you know, you know," Thomas says with a laugh. "I don't think she really expected any of those people [in Alaska] to know what she was ever wearing. The yellow V-neck Dries Van Noten sweater and the black pants is where she starts to loosen up just a little bit."

But Margaret does let loose ever so slightly as she falls in love with Andrew over the course of their short weekend together. Thomas admits, though, that even in the end, "she's still Margaret," and her wardrobe still reflects that.

"She's still put together, but she's a little softer and a little less severe."

Related: Sandra Bullock Recalls Silly Moment Doing Nude 'Proposal' Scene with Ryan Reynolds: 'I Didn't Look!'

With Reynolds, Thomas says it's "impossible" to make him look anything other than "handsome," and it was especially fun to put him in a suit for his assistant role. She says her biggest challenge at the time, though, was finding the right denim for him during the Alaska scenes because he's so tall.

"At the time there wasn't a lot of great denim with long inseams," she says with a laugh. "I imagine it's a bit easier for him now. He's got really, really long legs! But the Alaska scenes were fun with his wardrobe because we wanted to make him look comfortable because he was supposed to be around family and friends, plus he was doing masculine things like chopping wood."

Related: Ryan Reynolds Says 'the World Looks Different Now' Following the Death of Betty White

Looking back on the film now, after 15 years, Thomas says she loves that the clothes and the movie feel "timeless."

"Sometimes you look at rom-coms and you can date the fashion, which is not a bad thing," she muses. "That's rom-coms. They don't have to disguise that. But I do think this movie really is pretty timeless, which I love about it. A timeless movie with a magical group of people."

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