During the war, the Soviet entry on the Allied cause produced propaganda films such as Mission to Moscow, but the moment didn't last. Casablanca appeared in 1942-3: by 1947 its authors had been fired for their 'premature anti-fascism'. Reagan was an FBI informer from 1941, and I Married A Communist (1949) was a plot to expose 'pinko' directors: Howard Hughes made sure that the 13 directors who turned down the job had their careers wrecked (part of the reason why British TV and film was so strong in the 1950s was the influx of blacklisted writers, actors and directors).
Individuals were driven to inform on spouses and friends - some killed themselves or were driven out of town. Richard Collins co-operated with the witch-hunts: his ex-wife Dorothy Comingore was accused of letting her children play in a Communist's swimming pool and lost custody, before being framed on a counterfeit money charge: she was later framed for 'soliciting' and also spent time in a mental hospital, before spending the rest of her live in poverty and obscurity. John Garfield, strong-armed into co-operating with the FBI, finally refused to testify against his own wife, and died of a heart attack at 39.
One thing the FBI was skilled at, was identifying good films as liberal (to me, the two go together). Mr Smith Goes To Washington was 'decidedly Socialist in nature', while It's A Wonderful Life was 'an obvious attempt to discredit bankers' (Jimmy Stewart's George runs a building society which stands up for the little people discarded by the vampiric bankers). Clearly for the Feds, any critique of actually existing capitalism - however sentimental - was Communist subversion. Certainly hardline Republican Stewart would have been surprised, to say the least, that these films' sentimental populism was disloyal.