A lot of actors have no grievance with being typecast. If they have a certain physical look or type of voice that lends itself perfectly to a type of role -- and if that role turns up frequently enough that they can make a living playing it-- they'll just go with it. B movies and TV series need large amounts of sympathetic judges, worried doctors, no-nonsense generals, hypertense editors, nosy neighbors... the list goes on, and many actors spend their careers comfortably showing up with their lines memorized and getting a paycheck.
Of the two actors here, Johnny Weismuller (on our right) started as an award-winning swimmer who lucked into the Tarzan role for MGM and made it his own. He didn't spend years doing summer stock or learning stagecraft in off-Broadway shows, and when he grew too fat errr mature for Tarzan, he shifted over to the Jungle Jim series for a few more years in front of the camera. (Now, I'll watch cheesy movies until they make me constipated, but I draw the line at the Jungle Jim franchise; they're below the minimum entertainment I need to keep watching. When you see the same stock animal footage shown twice within a few minutes, it's too sad.)
Next to Johnny is, of course, George Reeves. Now, he was a different story. He paid his dues as an actor, working through dozens of small parts in big movies and big parts in small movies. Yet now, when you're sitting through an old flick on Turner Classic Movies with Jimmy Cagney or Tyrone Power, and George Reeves walks onscreen.. well, he IS Superman. Those voice, the face, the expressions, it's just hopeless. If he had lived on to grow a bit older and establish himself in different roles (the usual method for fighting typecasting is to take a diametrically opposite role), he might have gone on to a comfortable career and later in life be interviewed about those old days as the Man of Steel. But we know that did not come to pass.