The thing with Bush is that he didn't used to be that stumbling of a speaker.
Seems completely compatible with someone struggling with a script. I wonder if he got to the point where he figured he already had the job and since Brando didn’t need to learn lines why should he? One of my acting teachers would have us make dialog outlines of scenes. Their view was isn’t wasn’t sufficient enough to learn lines, but you also needed to understand what the point each character was trying to make. He’s tripping over words to the point where it seems his focus is on delivering words and not in conveying ideas.