most memorable 'unmemorable' character
... is Mr. Peacock. It's hilarious how obviously persistent Ford was in depicting this supporting character - he is in almost every scene utterly dismissable, but yet something in those scenes makes him one of the most memorable "unmemorable" characters in my opinion. Just remember how everybody confuse him with Haycock and whatnot... His peak in this movie is when he gets wounded by an Apache arrow - the very first sign of their attack.
I love mr. Peacock for being so completely useless the entire movie - he has almost no lines that have any importance at all, and he is, from the start of the movie, bound to be forgot. Yet, he is one of the Stagecoach passangers and this 5th wheel feeling that he gives makes the plot all the more dynamic in my book.
In my country, it's called "playing the stone" - actors who are noticable by doing absolutely nothing at all and still stick in viewers' eyes.
Just my opinion.