Why books go back on the shelf unfinished: the importance of good dialogue

I've been reading a mainstream fiction novel recently by a mature author. Its intriguing premise, recently pregnant girl washed up on a beach with no memory and no baby, captivated me enough to buy it from my local charity shop the moment I saw it. Sadly, it will be going back to the charity shop unfinished. Why?

As good as the premise and plot are, I just couldn't enjoy reading the novel without constantly frowning at the dialogue. With young twenty-something characters the novel, I think, is aimed at the 20-40 age range. Though the premise is good and the plot absorbing, the characters' dialogue tends to reflect the age of the author as opposed to their own. It can be incredibly frustrating to find yourself doubting the way a character speaks and thus detaching yourself from that wonderful "immersed" feeling we all enjoy when engrossed by a novel.

As a script reader I often see problems with dialogue. Most commonly is the "one voice" phenomenon - cover the characters' names up and you can barely tell one from the other. This also occurs in novels - the scriptwriter or author have either imposed too much of their own voice on their characters, or they haven't invested enough time in properly getting to know each of their characters; their accents, intonations, tone of voice and habits but to name just a few distinguishing traits that may help with characterisation.

The importance of realistic dialogue and characterisation can often be overlooked in favour of plot. Good narration and dialogue will immerse the reader in the lives of the characters and enhance the plot. A reader needs to be able to believe in the characters and the way they speak and behave. There should be no room for doubt - doubt in a reader, especially at manuscript/early script stage, could greatly affect your chances of success.