According to the IMDb trivia page, The Decoy Bride only had thirty-five percent the budget it needed for the original version of the screenplay, which—percentage-wise—is a default fail. Of course, it doesn't have to be; there are many examples of constrained budgets leading to ingenious filmmaking. Unfortunately, The Decoy Bride is not one of those examples.
The film's budgetary constraints are clear from the start when the opening titles repeat the scene just before them. Famous Hollywood movie star Alice Eve wants to get married, but since she's the most famous person in the world, the paparazzi are after her, and she can't get married if they take a picture. So she sends out decoy brides on her way to the chapel.
Later in the film, when they actually say "decoy bride," there's no acknowledgment of this opening gambit, which is either a gaffe or bad writing. It's one of the few times one can ask that question; usually, it's just bad writing.
The "Decoy Bride" is Kelly Macdonald. She's just returned home to her remote Scottish island of Hegg with her tail between her legs, yet another relationship failed. Her mom (Maureen Beattie) runs the only bed and breakfast on the island, so at least Macdonald's got somewhere to stay. The Hegg connection to Eve's character is fiancé David Tennant's plodding, way too long debut novel, which takes place on the island. Eve loves Tennant for the book (which no one else has ever liked, including Tennant). There are discarded subplots about Tennant not knowing what he's talking about with the island and even implying he may not have written all of it. There's no budget for a supporting cast, so it's not like he's got a Gordon Lish behind the curtain.
Macdonald immediately goes back to work for slightly creepy James Fleet, who keeps suggesting to her they need to marry (no one else on Hegg is unmarried), and Macdonald never picks up on the hints. Because bad writing. But he's important because he convinces Macdonald to write a travel guide for the island. A "marketing" conference is coming to town, and he's sure they'll buy the guide. So she writes and publishes it in less than a week or something. The travel guide gets discarded. The movie didn't have the budget for photocopies.
The guide's only necessary because after Macdonald and Tennant meet, she tells him she's an author too, and they trade barbs because it's a rom-com, and they can't like each other at the start. Especially since he's about to marry Eve, the most desirable woman in the world.
Most of the movie is set over a day when Macdonald's got to play decoy. It should be a comedy of errors, but they don't have the budget. Eve disappears for most of the second and third acts, only popping in to comedically threaten to murder someone in a wheelchair. To be fair, that part's the worst gag and worst acting in the movie. Well, wait, there are a lot of sexist jokes for a while, but for unnamed supporting players, the attempted murder is the worst for the main cast.
Speaking of unnamed… the film's got numerous characters who don't get proper names, including Sally Phillips, who plays Michael Urie's assistant. Urie is Eve's assistant, though I don't think they have any scenes together. Urie's actually an American playing an American, which is too bad; a Brit doing a bad American performance makes up for a lot of his performance. Being American, he's got a lot fewer excuses.
Except, of course, that bad writing. And director Folkson doesn't do her cast any favors.
The movie somehow manages to waste Tennant's charm (for large stretches, anyway), and then Macdonald is one of those female protagonists who are also the butt of the jokes (can't get a husband, can she). Eve's woefully miscast. The most damaging performance is probably Beattie.
In addition to the severely wanting script, Folkson's direction is barely middling. The quaint, remote island has no personality. The recurring gag is there's a relatively ancient public toilet. So if it's not funny the third time, what about the fourth. Wokka wokka.
The third act seems like it might rally and surprise, then hits all the predicted beats instead, which is the film's final disappointment.
There's also the soundtrack, which frequently features cloying, overbearing bland folk-rock, set to unrelated scenes for the entire song. Then the score's main theme references "Just Like Heaven" so much you'd think someone told them the Cure would definitely let them use the song for free. And then when they did not, the movie just left the theme because it's not like anyone associated with the Cure would watch the film?
It could be worse, obviously. But almost anything would've made it better. Just trusting Tennant and Macdonald to act instead of blaring crappy music over their scenes would've done a lot. The film doesn't trust its leads, which is the entire point of a rom-com, so why bother.