Ken Levine, who knows of what he speaks on baseball and comedy writing, writes about the latter…specifically, how actors can and should suggest changes in a script in a constructive, non-tantrum manner.
You do get, especially from seasoned amateurs, a lot of "ego" notes where someone pretends they're just concerned with the health of the show…but what they really want is a bigger part or to lose that joke that suggests they're getting chubby. I've worked with actors who seem to find all sorts of structural flaws in any script where someone else gets a laugh.
The big problem is usually a performer who's way too quick to say of the material, "This needs a rewrite" or, even worse, "I have to save this." In the seventies, a lot of sitcoms were harmed, I believe, by all those articles that said the cast of All in the Family was tossing out the script each week and either demanding a new one or improvising/writing in their rehearsals. That seems to have been true at times on that show but it led to actors on many sitcoms thinking that your final draft was merely their starting point and it was up to them to start rejecting material and maybe rewriting on their feet…and the sooner they got this process started, the better.
That kind of thing becomes self-perpetuating. It can lead to the writing staff deciding, perhaps sub-consciously, not to put as much effort into the scripts that are handed out on the first day of rehearsal, and to save the effort (and good lines) for the drafts that will be generated later in the week. On one show I worked on, if you came up with a good line early in the process, the producer would say, "Save that one 'til the day before we tape." Because if you put it in too soon before then, it would never make it to Tape Day. Weaker table drafts, of course, merely heighten the actors' feelings that they need someone — themselves or the staff — to do rewrites. Rightly so.
There have been, of course, shows that were famous for trusting the writers and sticking largely to the script as written. M*A*S*H appears to have been one, and no program was ever more successful. (Then again, All in the Family was no flop.) Some actors are good at spotting valid weaknesses in a script and some, quite frankly, aren't. There was an interview once with Donald Sutherland where he said that for the first decade or so of his career, he'd argue over every line with the director and/or writer. Then at some point, he decided — just as an experiment — to skip all that and just do the lines as written and directed. His ultimate conclusion was that it made no difference. The final product was no better or worse because of it.
Anyway, in his piece, Ken cites the actor Nick Colasanto as having just the right approach to suggesting that his lines could use some work. The best I ever encountered was when I did a show way back in the mid-seventies with Eve Arden. She only said it twice to me but I gather it was her standard line when she thought her part needed another pass through the typewriters we then used. She said, "I'm sorry. I can't make this work. You're either going to have to teach it to me the way you want it, rewrite it or hire a much better actress." Maybe it was how she said it that made us rush to rewrite. And then she was genuinely grateful for what we gave her instead.