Sometimes it's a bad performance. Sometimes it's a scene or an ending that seems completely out of place. These are just some of the elements that can reduce a potentially great movie to merely a good one, or an OK movie into a bad one. We're talking the "Greedo shoots first" nonsense George Lucas inflicted on Star Wars: A New Hope. Or the staging of the big reveal in the original Planet of the Apes.
The last-minute casting of Sofia Coppola (the director's daughter) in The Godfather Part III after Winona Ryder dropped out has long been a target of derision. Sad to say the criticism was well deserved. Her performance was wooden - she is far more talented behind the camera or as a writer and director as films like Lost in Translation demonstrated.
But another flaw was not a presence but an absence. The producers could not come to financial terms with Robert Duvall and the absence of Tom Hagen is glaring. In the earlier films the death of one of Michael's brothers was pivotal and we can only wonder what would have happened if Duvall had been in this film, which did not measure up to the standard of its predecessors.
Bram Stoker's Dracula is the Francis Ford Coppola film in which Ryder did star. It was one of Coppola's best films made in the latter part of his career, with excellent costumes and production design, but Keanu Reeves seems to be concentrating so hard on maintaining an English accent that his performance is awkward and stiff. Reeves as Don John was also the worst part of Kenneth Branagh's Much Ado About Nothing. He's better suited to action roles.
Quentin Tarantino started out as an actor but, like Sofia Coppola, his real talent is behind the camera. Tarantino's hammy performances in Pulp Fiction and Django Unchained were low points in otherwise good movies. A cameo is one thing but smart directors who know they can't act keep it brief (see Alfred Hitchcock).
Driving Miss Daisy had excellent performances from Jessica Tandy and Morgan Freeman and even Dan Akroyd was quite impressive in a rare straight role (despite some crude ageing make-up). Hans Zimmer's music wasn't bad but the film's low budget apparently meant it couldn't afford an orchestra or small band, so it was electronically performed, which felt jarring and anachronistic.
Life is Beautiful did have an excellent, well performed score: its big problem was the second half, when the lighter, romantic side of the film gives way to Meaningful Holocaust Drama and the story becomes completely unbelievable. And Roberto Benigni's shtick got old fast.
The Tobey Maguire Spider-Man movies were fun but burdened by a miscast lead: as so often, he seemed self-conscious, acting outside himself, and Peter Parker's angst never really registered. Even if you liked Maguire, however, the third in the series, besides being overlong and overstuffed, had his infamous, ludicrous dancing.
On stage or screen, the work of lyricist and (sometimes) composer Leslie Bricusse is frustratingly inconsistent. Sometimes it isn't his fault: the actor performing the memorable Candy Man in Willy Wonka & the Chocolate Factory isn't much of a singer.
Pure Imagination has a rendition worthy of it but Cheer Up Charlie is a cloying song that holds up the action (comedian John Mulaney called it "an awful number - we all fast-forward through it" and even the director said he occasionally removed it for TV screenings).
And I Want It Now makes Veruca Salt the only one of the factory-touring kids to get a number (and a very annoying one).
Sometimes the ending is the problem. The Lord of the Rings: The Return of the King seemed like it had reached a satisfactory ending, then there was another ending, and another ending ... Peter Jackson seemed to have trouble letting go of anything.
Sometimes it's the beginning. Saving Private Ryan's bloody, visceral landing scene is justly celebrated, but it's preceded by a bland, unnecessary prologue. And the epilogue feels a bit lacking too, somehow: while the point about sacrifice and loss is made, we're never told anything about what Ryan did to, as one of his rescuers told him, "earn it".
Having a bad element or two doesn't necessarily mean the whole film is ruined. But it can certainly frustrate and disappoint.