The biggest question always seems to be about not falling through floors...
But if Sam and other spirits cannot interact with the physical world (without a skill) this would apply to everything, not just doors. Example: Sam followed Willie onto the train. How? A train itself is a physical object. What keeps Sam onto a train, and traveling at train speeds?
kylopod answered this 7 years ago
This is basically explained in the movie itself, during the Subway Ghost's speech:
"The problem with you is you still think you're real. You think you're wearing those clothes? You think you're crouched on that floor? Bull****! You ain't got a body no more, son! [taps head] It's all up here now."
In other words, Sam isn't physically there. His entire appearance that we see--his wearing the shirt and pants he was killed in--is nothing more than his own mental projection of himself. He looks the way he expects himself to look, and he's subject to the rules he expects to be subject to. He doesn't fall through the floor because he doesn't expect to fall through the floor. He isn't subject to the laws of gravity, but he is subject to his expectations of the laws of gravity.
The only remaining question, then, is why he moves through objects he attempts to move--doors, lamps, soda cans, and so on. Wouldn't the objects simply stop him in his path, the way the floor keeps him grounded? Of course the movie is simply following the common conventions about the way ghosts experience the world, but if you want an in-universe explanation, I suppose the answer is that his attempt at conscious control over the objects is what stymies him. He tries to move an object, but he can't, and he experiences this limitation as his hand passing through the object. But when he stands on a floor, he isn't trying to move the floor; he's simply experiencing reality the way he was used to when he was alive.