THE KREMLIN LETTER always seemed to me an underrated film. The ending was said to be preposterous, but really, knowing what we do now about Cold War machinations, the whole thing seems to be within the realm of possibility. I once fell in with a group of "International Couriers," very popular in the 1980's. The leader, who spent some time with me (wanted to recruit me!), thought both the novel and the film among his favorites. He liked the novel a tad better because he could savor the details, but he thought the film one of the best spy pictures ever made.
He said that the plot, as he knew something about the life, was drawn from reality, including -- in an exaggerated sense -- the ending!
Welles has a supporting but important part.