Hi
In the pen picture it stated that he oly appeared once for John Ford and the two didn't get along.
Apparently in My Darling Clementine in one scene Brennan had to mount a horse very quicky a scene that he had difficulty in doing as he struggled to control the horse. Exasperated Ford exploded what sort of western actor can't get on a horse to which Brennan replied the sort that has won three Oscars.
No wonder that he only appeared for Ford once.
Regards
Arthur