In Marathon Man (1976), the characters’ outward benign appearances mask their true potential for danger.
We have Dr. Christian Szell (Laurence Olivier) looking like a harmless old man, exhibiting an almost passive exterior. In reality he is a former Nazi dentist/torturer who experimented on prisoners at a WWII concentration camp. Szell earns his living by selling diamonds from wealthy Jews who wished to escape Nazi Germany. He has a spring blade dagger attached to his arm under his coat which he uses to slash away at anyone who tries to expose him. Medical practitioners, on the surface at least, follow the medical axiom "to do no harm,” but that rule is alien to Dr. Szell.
To add to this ironic medical reference, Thomas Levy (Dustin Hoffman) has a brother Henry (Roy Scheider) who is called "Doc." He is no healer. Supposedly a businessman, he is actually an espionage agent working with Szell and his network of jewel traffickers. Another poser is Janeway (William Devane), who, as Henry's boss, pretends to aid Thomas but then double-crosses him. Even Thomas’ love interest, the beautiful Elsa (Marthe Keller) turns out to be working against him.
Hoffman's character is a nerdy graduate student doing his doctoral thesis on the effect of McCarthyism and how it destroyed his father. Hoffman is not an imposing figure physically, so he does not appear like someone capable of dangerous acts. His nickname is “Babe,” which implies that he is a naïve innocent. In the climactic scene between Thomas and Szell, the Nazi accuses the former of being weak “like the rest of his family.” But as it turns out, Thomas is anything but weak. He is a marathon runner who lives by the philosophy he articulates earlier in the film: “You forget about the pain while striving toward the finish.” Even though Szell has caused him excruciating agony by drilling into his teeth, Thomas throws away the pain-killing essence of cloves. He does not want to be numbed into passivity but lets the pain keep him sharp. He guns down Janeway when the spy, trying to kill Thomas, accidentally shoots Elsa. He confronts Szell at a pumping station, and tells Szell he can keep as many diamonds as he can swallow. During the physical confrontation that follows, Thomas dumps all of the diamonds and Szell falls on his own blade after diving for the jewels. Despite appearing to be a nonviolent person, Thomas is the only character who survives and triumphs in the end,
This film makes the statement that underestimating the dangerous nature of individuals by observing benign outward appearances can be deadly.