The physical appearance of the actors playing the characters in Washington Square is not necessarily like the picture one visualizes while reading the story. Catherine's character in the movie is not as overweight as it appears in the book. She is actually attractive in the movie, and this might take away from the emphasis on her average look that one would read in the novel. Morris is said to be handsome in the novel, but his character in the movie is exceptionally good-looking.
Some characters' personalities come across differently in the book and film versions. In the movie, Olivia de Havilland's Catherine, for example, had more of an airy personality than she did in the novel. Her facial expressions and attitude towards Morris (Montgomery Clift) were different from what were described in the book. She is more composed in the novel. Also, she seems not as "unlearned" in the movie because she is clever enough to set Morris up by faking her desire to reenact their elopement and then leave Morris pounding in vain on the locked door at the end of the movie. Another example lies in Morris's personality. In the film, he seems less composed and more focused on the idea of courting Catherine than what some might have found in the book. He seems truly interested in Catherine in the film. which is shown through his facial expressions and body language, where in the book one might get more of the sense that he is merely after her father's inheritance because he does not show affection to her as sincerely.
To some these differences may not seem important, but they could change one's entire opinion of the story at hand. Someone's perspective of the character's personality in this novel is vital to grasp the full effect of the story. Morris and Catherine's looks and personality were obviously different in the film compared to the novel. This is just one more example of how a great novel can be differently represented by producing a film about the novel.