take a step on the door
From the movie "The Secret Bride" (1934)
In the beginning of the movie characters of Barbara Stanwyck and Warren William get married. After that they come out of the priest's office and here a dialogue goes:
WW: Well, how does it feel?
BS: I don't know. It's a little like taking a step on the door thinking it's a short step.
WW: And finding out it's the elevator shaft.

The line of Barbara doesn't make any sense to me. What's "step on the door"? Meaning "inside the door"? Why THE door since they are obviously talking about some hypothetical things. And what's "a short step" here? What does the line mean?

Question

take a step on the door
From the movie "The Secret Bride" (1934)
In the beginning of the movie characters of  Barbara Stanwyck and Warren William get married. After that they come out of the priest's office and here a dialogue goes:
WW: Well, how does it feel?
BS: I don't know. It's a little like taking a step on the door thinking it's a short step.
WW: And finding out it's the elevator shaft.

The line of Barbara doesn't make any sense to me. What's "step on the door"? Meaning "inside the door"? Why THE door since they are obviously talking about some hypothetical things. And what's "a short step" here? What does the line mean?

Answer

Are you sure she didn't say "a step out the door"? That would be a short version of "a step out of the door"
It would sound odd to use "a door".

She probably means a step outside. (into the unknown)

Answer

'Step on the door' would be incorrect English.  
As explained by the other person, it should be 'out' the door. 
It simply means that she thinks it is a small thing, but when she takes the step of getting married, she find out it is very big indeed.