what is the change in macro expansion when using environ and newenviron as compared to the standard environment

Question

The packages environ and newenviron both provide an alternative to the standard environment macros included with LaTeX. The main difference is that they make it possible to refer to the environment body using a macro, but another difference is that macro expansion is affected as can be seen by (un)commenting the relevant lines in the following document:

\documentclass{article}
\usepackage{environ}
\usepackage{newenviron}

\def\speakify#1: #2\par{{\bfseries#1}: {\itshape``#2\unskip''}\par}
\newenvironment{interview}{\everypar{\speakify}}{\par}
%\NewEnviron{interview}{\everypar{\speakify}\BODY\par}
%\newenviron{interview}{}{\everypar{\speakify}\envbody\par}

\begin{document}
\begin{interview}
I: I have a question for you that I would really like to know the answer to. Could you please explain it in your own words while the moon is still high in the sky? That would be lovely thank you very much!

L: Of course I will answer gladly all questions you might have no matter how silly I feel they may be provided payment is made in full before commencement of the answering phase.
\end{interview}
\end{document}

What exactly is the change in macro expansion that occurs?

Answer 1

After the \everypar processing of the second paragraph, with the document as posted you have essentially

\speakify L: Of course I will answer gladly all questions you might have no matter how
             silly I feel they may be provided payment is made in full before commencement
             of the answering phase.\end{interview} \end{document}

Now \speakify sees L as #1 but runs off the end of the document looking for \par to delimit #2. So you get:

! File ended while scanning use of \speakify.
<inserted text> 
                \par 
<*>

If you edit the document so there is a paragraph break after the environment so the file ends

phase.
\end{interview}

\end{document}

Then \speakify would include \end{interview} in #2 so the above error is avoided but the grouping logic is messed up and you get:

! Missing } inserted.
<inserted text> 
                }
l.16 

? 

If you edit the document to put a paragraph break before the end of the envitonment, so the file ends:

 phase.

\end{interview}
\end{document}

Then the document works without error.

---

The expansion logic is different in the forms using \BODY as you have added an explicit \par so \BODY is expanded and when the final paragraph is processed, \par is already in the input stream so the processing is equivalent to

\speakify L: Of course I will answer gladly all questions you might have no matter how
             silly I feel they may be provided payment is made in full before commencement
             of the answering phase.\par

and \speakify sees its argument delimiter as intended.