I have recently spend many days in casting for a show. This is a fairly new experience for me. I've done some casting before, but this amount casting this many parts is new to me. Without dwelling on the details of the show and the people we saw, here are some comments about what the write can gain from casting - and some things to look out for.
The Joy of the Jokes
One of the most exciting things about casting is hearing dialogue read aloud by someone who knows what they're doing. A script that has been sweated over and tinkered with for weeks, months, or even years, is finally being taken for a brief test drive, by someone who at least knows how to drive, even if they're not the Formula One star you'd been hoping for. Even though you, the director or producer are reading in other parts, just hearing it read aloud is a wonderful experience. You hear some of the jokes, which is a relief, but the process also exposes hitherto hidden flaws: lengthy dry patches without jokes; lines that are funnier than was first thought; and bits that just sound wrong or confusing. After a few days of various actors reading in different parts, my writing partner and I found there were already enough issues raised to produce a new draft of the script. This made the script stronger. Rejoice.
The Refinement of the Character
Seeing and hearing a decent actor read a part will help you define the voice. Sometimes, you think a character should be played posh, but when you hear it read posh, or very posh, the character becomes insufferable and you've learnt something. Or an actor reads a part in a completely unexpected way that sounds wrong, but it's good to hear it that way because it confirms what you already thought about that character. Or opens up a new direction. This is hard to conceive because lots of writers have a very fixed idea in their head of what a character should look like and sound like - and they can expend much energy arguing for someone who most fits that brief, even if they're not the funniest or the best. In reality, another actor might breathe life into that part and make it more interesting, much as a writer would hate to admit this.
One for the Back Pocket
You often see actors who are funny and interesting but not right for the parts on the table. But you need to remember these people, because you might need them in the future. When you're writing a one-off part, or even a new character, in a future episode or a different show entirely, your writing partner or director might say "Hey, remember that actor we saw? With the odd hat? Who did that thing? They really had something about them. Could this character be right for them?" And then you're creating a part that you're pretty sure will be brilliantly executed which, in the chaotic throes of a TV series, is a useful short cut to the funny.
The Stars Are In The Sky For A Reason
Writers are drawn to unknowns. For several reasons. One is that unknowns have less experience and therefore more likely to do what they are told, and perform the show as written. (Writers like that). If/when the show does become a success, the script stands a greater chance of shining because critics are less like to attribute the success to a new actor (Writers are obsessed with critics, even though they think they're idiots). Writers are not attention seekers, and tend to think less of people who seek the limelight. Stars are very happy with attention, and writers tend to think less of them for this. But let's be clear about this. Stars are often stars because they're really good. And when they audition, they tend to sparkle. An experienced star brings more than just their name and reputation to the show. They bring so much more.
Overall, the casting process is a useful reminder to the writer that, much as they would hate to admit, comedy is a collaborative process, especially in television. An good actor brings experience and insight to this creative process which I hope, in our better moments, we would acknowledge. The best shows are usually a happy accidents of teamwork, rather that the ruthless execution of one person's vision.