One of the article links below faults "Method" acting, a meaningless term, and goes on to fault Marlon Brando in particular for being a "proponent" of this trend. The writer had apparently not viewed Brando's performance as Mark Antony in Shakespeare's Julius Caesar, and referred to Guys and Dolls and The Godfather. Even with cotton in his mouth as Don Corleone, Brando can be heard and understood. To suggest that he did this to make himself less intelligible is uninformed and misleading. He once credited Stella Adler with having taught him everything he knew about acting, and she taught that "...there are no throwaway lines."
As I wrote in my original post on this subject, not being heard or understood is related to the actor's not understanding the importance of playing the action. In addition, I'd say the importance of needing to make one's partner understand, see what one sees in their mind's eye; of the actor understanding the importance of the progression of each succeeding sentence that the character utters.
I think the mumbling (as well as not pronouncing consonants*, or contracting and connecting individual words and sentences) is due to the actor not having learned the basic technique, and is related to my other post and article in Backstage regarding the error of isolating the individual exercises of Strasberg, Adler, Meisner or Hagen, incorrectly naming them techniques, concentrating on mainly those exercises, and giving them precedence over the fundamentals of the technique the Group Theatre adapted from Stanislavski.
I've had enough foreign students to know that the Group Theatre teachers' exercises are taught in many countries. If the bad habit of perpetuating dogma in place of curiosity and inquiry continues, acting students will continue to be deprived of the opportunity to fully realize the nurturing of their talent.
*Robert Lewis pointed out that if the actor says, "I killed him" without pronouncing the d, he changes the plot.