I've probably mentioned once or twice, here and there on this blog that one of my sisters works in film. Like all my siblings, she's very good at what she does -- one of the live-action short films she produced was shortlisted for this year's Oscars -- and I believe we'll see even better things from her in future.
We talk sometimes about the details of film production (because, hey, I think it's interesting stuff), and on more than one occasion we have talked about what can be done when someone has made a serious error in casting for a film. Miscasting is potentially a bigger problem in an indie production than it is in a big-budget film, because indies are usually driven by specific performances and produced on a shoestring budget; if something goes seriously wrong with an actor's performance, there's usually no extra money to recast the role or to go back and get better footage -- especially if the production was filmed on location on the other side of the world. Minor problems can sometimes be fixed in post-production, but sometimes the situation is so bad that you must edit out or edit around a particularly bad performance to get a viable film out of the footage you have. (This is why there's an Oscar category for Best Film Editing -- it really is an art.)
There are all sorts of reasons why film casting can go wrong. Despite their impressive ability to pick both well-known stars and brand-new talents for roles, casting directors are just as human as the rest of us and they do occasionally make mistakes. Sometimes they're temporarily dazzled by the personal charm of an actor. Sometimes they're so keen to get a big-name actress in the production that they don't consider whether her specific acting style suits a given role. Sometimes an actor auditions at the very height of his powers, and cannot summon his A game again throughout production. Sometimes a personal tragedy derails an otherwise excellent actress's abilities. Sometimes an actor is a personal friend of the director, who owes his buddy a favor. And sometimes, you have to admit, the only reasonable explanation is that everyone involved with the production was smoking crack.
Of course, the other big reason for movie musical failure is that some actors can't sing. Surprisingly, sometimes this failing doesn't make much of a difference. Rex Harrison rhythmically chatted his way through most of the musical numbers in My Fair Lady, and his performance was such a hit that virtually all other stage 'Enery 'Igginses ape Harrison's interpretation of the role. But for the most part, if an actor is to be successful in a musical he has to be able to a) stay in tune, and b) interpret a song with the same skill, thoughtfulness and depth of feeling he draws upon to interpret his character. Just because you're modulating your voice to hit a particular note doesn't mean you stop acting; if anything, singing in a film requires even greater acting chops, because you must help the audience trust you enough to decide to get over the intrinsic unnaturalness of what you're doing and just go with the flow.
Don't misunderstand me -- I don't have an axe to grind against this particular actor, who has made some good films in his time (and will probably make more unless this performance turns out to be a career killer). It's just that he was manifestly, painfully wrong for the part.
Here's an analogy: let's say a friend of mine is directing a world-class, definitive film version of Shakespeare's Macbeth. He scouts out gorgeous locations in Scotland, gets a fantastic cinematographer, manages to gather a stable of A-list actors for every role -- and then, presumably because my friend has a serious meth habit, he chooses to cast me as Lady Macbeth. Now, I've done a few plays here and there in college, and I've been told I'm a pretty good little actress; it would be very tempting to be offered a role in a production of that magnitude. But if I were to accept the role, I'd be working with the best of the best the cinematic world has to offer, and my relatively meager acting talents would be constantly compared to theirs and found wanting. As much as I might want the role, I wouldn't be the right choice for it at all. I probably wouldn't even have the right body type to be cast as a background actress. If I had a lick of sense and the desire to help my friend make the best film possible, I'd turn down the role. And then I'd urge him to get into rehab.