Throughout his career, Brennan was frequently called upon to play characters considerably older than he was. The loss of many teeth in the 1932 accident, rapidly thinning hair, thin build, and unusual vocal intonations all made him seem older than he was. He used these features to great effect. In many of his film roles, Brennan wore dentures; in MGM's "Northwest Passage" (1940) – a film set in the late 18th century – he wore a dental prosthesis which made him appear to have rotting and broken teeth. Brennan was billed third in "Northwest Passage" after Spencer Tracy and Robert Young.