Myth: It’s better to get the lines and blocking down first, and then add the dialect once you’re comfortable with everything else…
Truth: In film and TV this isn’t even an option. Rehearsals are limited and every minute on set costs the studio money, so it’s essential that you are 100% performance-ready with any dialect (or other skills you might need) the moment you first arrive on set. Actually, if you aren’t performance-ready with your dialect well before you even audition, you probably will be passed up for the role entirely.
As for theatre productions, it’s important to keep in mind that the very sounds you utter onstage are a significant part of your character’s life moment to moment. If your dialect is ‘in-progress’ during rehearsals, you will never be free to really connect with other actors and rehearse. Half of your mind will be always be elsewhere thinking ‘Whoops! That was a dialect mistake!’ or ‘Yes! Got that one right!’
Truthfully, if your dialect is not ready for integration at the first read-through, the entire rehearsal process is going to feel awkward and uncomfortable. Of course, it is this discomfort that makes it tempting to want to put off the dialect work in the first place, but actors who choose that route find themselves in the 11th hour suddenly forgetting blocking, dropping lines and never really being able to just let go and act.
In a production where the dialect work has not been incorporated on time, the result is most often that an inconsistent and distracting attempt at the dialect diminishes what would otherwise be a solid show. Even if the production manages to escape earning a scathing review, you won’t find audiences rushing to see it.