Like many other authors, I often "hear" this sort
of "under the stress of emotion" change of diction
or accent or dialect when the character is speaking in my head,
and put it into a character's written lines.
I do it subtly, w/o attempting to indicate it phonetically,
and sometimes wonder whether to call attention to it with a
stage direction-- but I usually decide not to. I figure, if
the actor is sensitive to verbal nuance, s/he'll pick it up
and do it, easily and naturally. If the actor has to make a
conscious effort to "achieve an effect", the effort
will call attention to something which if successful should
operate under the audience's radar. They should get the impression
that the character is falling apart without noticing the specific
symptom that clues them in.
by 
