AYes. Hitchcock did a cameo in each of his movies from Rebecca (1940) onwards. His cameo in Marnie appears about six minutes into the movie. As the bellhop and the woman with the black hair (who turns out to be Marnie) walk down the hotel corridor to her room, Hitchcock can be seen on the left, coming out of his own hotel room. A photo of the scene can be viewed here.
AMark takes Marnie to see Bernice. A storm is raging outside, and Mark has to shelter Marnie to get from the car to the house. When they're finally inside the house, Marnie collapses on the staircase. Mark tries to force Bernice into telling what happened to Marnie when she was a child, but Bernice refuses to talk. Mark points out that he's hired a detective and read the court transcripts, so he knows that Bernice was once a prostitute. Bernice starts hitting Mark to make him leave. Suddenly, Marnie regresses to her childhood and begins to relate the story. One night, her mother had a sailor (Bruce Dern) as a client. Mother rapped on Marnie's bedroom door and made her move out to the couch so that they could use the bedroom. A thunderstorm was raging, and Marnie got scared. The sailor came out of the room and tried to comfort Marnie, but she didn't like his breath when he was kissing her. Mom came out and tried to fight off the sailor with a fireplace poker, but he turned on her and started hitting her. In an attempt to protect her mother, Marnie picked up the poker and hit him in the head with it. The blood came streaming down his face, killing him. Bernice testified that she was the one who killed the sailor in self-defense and fought to keep custody of Marnie. Marnie realizes that her mother really did love her and lays her head in Bernice's lap, but Bernice tells her to get off because it's hurting her leg. Marnie expresses her fear that she'll have to go to jail now and decides that she would rather stay with Mark. In the final scene, Mark and Marnie leave Bernice's house and drive away together.