1, The plot is for me the most important thing. So THOUGHT in a play decides everything: that may be probable music, costumes, the details of the plot, lighting etc. - everything follow afterwards.
2, For the detailed work on the plot, I think it is important to decide before the start of this work if the aim is an OPEN play (i.e. with many decisions up to the director etc.) or a DETERMINED play in which the author is very determined about the setting etc. (an example would be: 1960s, New York, Jazz music in a dimly lit bar, etc.)
3, An important thing is not only to create the CHARACTERS. It is also important to imagine them roughly and uninterruptedly to ask oneself: "Does the thing that the character does on stage, the thought he/ she has, the line he/ she speaks fit to his character?" I think that "mimesis" in the Aristotelian sense comes into (the) play here, just to narrow down the infinite possibilities a character could act like, to find a possible acting range for him or her. That is, to make it possible for the audience to follow the plot.
4, A play is not like reading a book in a silent chamber. Things can be shown in an allusional way and be entertaining and still carry serious content. Therefore, it is important as a playwright to ask oneself: "Can I use MUSIC (melody) here? Could I use NONVERBAL (paralinguistic) means to convey my plot idea (see 1,) to the audience? Is there any other means I can use (LIGHTING etc.)?" (An example are the mocked dance videos in the "101 ways to . . ." play of the New Works Festival).
5, Any play stands in the tradition of an oral and visual culture rather than that of scriptures (although the plot is the most important part). That means there is usually no possibility for the audience to follow very fine or abstract, infinitely philosophical etc. aims, but there is one idea or one story which is told. To use the power of the oral and visual tradition it might be useful to employ surprises and thus, create a SPECTACLE. But: It is important that the spectacle is not more important than the plot, that means it has to be used as a means to support it ("delectare et prodesse").
6, Any author has to think of the AUDIENCE he writes for. That is, because one play could cause ecstatic agreement or spiteful hatred or astonished non-understanding among its audience. An example would be to show the "101 ways to . . . " to 1,000 seventy-years-old farmers from Uzbekistan which would just not be understandable to them.
The DICTION of any play has to follow the plot like every other part of it, too. But still, there is one important thing: The playwright has to find his OWN STYLE OF EXPRESSION. In my opinion, this relates not only to the diction but can be any other part serving the plot (for example a theme announcing a certain character etc.) It is good to quote, mock or rely on any other part of cultural evidence, like Greek myths in "The Psyche Project". I think, if a playwright imagines the characters of the play and combines it with his/ her aim the style of expression or the diction follows up naturally.