Was just re-reading your previous post as I've just put on hold a sitcom idea to pursue a film idea that's been niggling away at me for ages. I was just wondering why you suggest developing a character who doesn't appear until the second act and one until the third. Is it important to bring in new characters later? Is it to keep the audience's interest piqued and prevent things getting monotonous? I have a couple of ideas for those characters having pondered it so thanks for the tip.Firstly, you don't always have to put one idea on hold in order to pursue another one. Obviously it depends on how much time you have to invest, but it's worth remembering that if do make a living out of scriptwriting, you'll probably end up having to work on multiple projects at any one time, at various stages of development, so it might be worth getting used to the idea now. On to the main point, I'm not sure it's important, or even necessary to bring in new characters later as such - it's more a reminder for me, as my instinct is to bring in all the characters at the start, make them jump through hoops and then wrap it up.
So I have to remind myself that you don't throw in all the ingredients into the chili pot straight away - sweat the onions first, then brown the meat, then add the chopped coriander and jalapeno right at the end where it'll have the most impact. I appreciate cooking metaphors for writing are a bit trite, but, you know, they can work.
It's also more that a character can represent a shift in the story: it's unlikely Walter White would have met dodgy lawyer Saul Goodman right at the start of Breaking Bad, for example, but him becoming a regular character shows the darker parth Walt is heading down. If Saul had been present from the start, that would have suggested Walt already having a foot in the criminal world, or you would have had a more innocent Saul being pulled down into darkness along with Walt - which is a journey Jesse is already on.
Or if you don't watch Breaking Bad, think how the introduction of Han Solo in Star Wars shows that Luke Skywalker is heading wayyyy out of his comfort zone. So later introduced characters are handy to mark how far a protagonist has moved on since the start of his or her journey. Dunno, is that pointing out the obvious? Possibly, but it's the sort of obvious of which I need to be reminded worryingly often.