Black Friday (1940)

A vehicular accident involving an English teacher and a gangster in which both lives are endangered is dealt with by secretly transplanting part of the brain of one into the body of the other.

Much too soon after screening The Man with The Screaming Brain, right?

Originally the teacher identity dominates the body; but for Boris Karloff, (initially a supposed kindly doctor,) to get a hold of the half a million dollars that were hidden by the gangster, that side must also be awakened.

Once that is done which personality will surface is out of anyone’s control; the gangster personality takes over at random times and starts going after his former colleagues.

In an odd bit of casting, the meatier, starring dual role was given to Stanley Ridges, and he does a damn good job with it with minimal physical alteration, (he loses the professorial glasses and slicks back and darkens his hair.)

Karloff and Lugosi (in a minor part as a gangster) are there mostly for support.

Despite the science fictional premise of a brain transplant this is basically a Crime feature. The brain transfer functions mostly as the old plastic surgery gimmick if one were to add a near Jekyll and Hyde aspect to it.

Screenplay by Curt Siodmak of later Donovan's Brain fame.