Harry Morgan, in his role as the town marshall, is downright embarrassing in this film. His so-so acting and ridiculous lines remind me of his light-hearted role as officer Gannon on the 1960s TV show Dragnet.
Harry Morgan is an experienced actor who's performed in a number of roles. He generally plays a cantankerous type with a comedic-edge in many of his roles, and was a bit over-the-top as the town Marshall. However, I believe it was done purposely to provide some balance to the grave tone of this movie concerning a man facing his own mortality.
Firstly, it's "marshal," not "marshall," no matter how it's listed in the script or cast list.
Secondly, I agree with the OP. Morgan is the weak link among the cast in The Shootist, hands down. It's almost like he's playacting rather than film-acting. This is one of my otherwise most favorite films and I try to give him a pass, but every time I see this film I am amazed at how bad Morgan is in it. And no, it's not just to to show that the character is a buffoon or out of his element or whatever. It's bad acting, plain and simple.
I remember the criticisms of his performance when the movie first came out; and maybe, influenced by that, I felt the same way when I saw it in the theaters. However, I watched the movie on GritTV yesterday, and I enjoyed his performance. It was nice comic relief. I could have done without the gigantic Stetson, however, although it fit his character.