Although I have not (yet) read his autobiography, from quotes I've read from Mr. Cotten (if accurate) it sounds as though he didn't take acting too seriously in terms of his own abilities, that it was more of a job to him, one he felt he could do successfully primarily because of his physical aspects. For example, from his IMDb page: "I was a so-called star because of my limitations and that was always the case. I couldn't do any accents. So I had to pretend. Luckily I was tall, had curly hair and a good voice. I only had to stamp my foot and I'd play the lead -- because I couldn't play character parts."
I don't find Cotten bland as an actor, although I do find he was sometimes cast to portray bland/weak characters. For example, in
The Magnificent Ambersons, Eugene Morgan is a rather bland man, one with little strength of character. Ditto in
Portrait of Jennie. They are men who are intelligent and capable, nice people, but are about as interesting as buttered toast. However, his Dr. Drew Bayliss in
Hush, Hush Sweet Charlotte has much more spice and texture.
Btw, a post above mentioned "The Best Years of Our Lives." Mr. Cotten was not in that film; the male leads were Fredric March and Dana Andrews.
Most of the Cotten films I've seen are the ones he made with AM. I should make an effort to see more of them.
He worked a lot on stage too, throughout the years, sometimes co-starring with his second wife Patricia Medina.
A preview of his autobiography, "Vanity Will Get You Somewhere" is available from
Google Books.