- I will come in? / - Please do.

[Uncanny]

Is this dialogue idiomatic:

John (having opened the door a bit): I will come in?
Jill: Please do.

 

[emsr2d2]

Is this dialogue idiomatic: grammatically correct and natural?

John (having opened the door a bit): I will come in?
Jill: Please do.

No. For a start, John's line needs to use the correct word order for a question. However, note that BrE doesn't use "Will I ..." for a question. We would say "Shall I come in?" I don't know about AmE. I do know that many people in Ireland would use "Will I come in?"

 

[SoothingDave]

"Can I come in?" is likely in AmE.

 

[emsr2d2]

"Can I come in?" is likely in AmE.

That's also very likely in BrE. I only suggested "Shall I come in?" to stay as close to the original use of "will".

For the benefit of the OP:
"Can I come in?" asks for permission.
"Shall I come in?" makes a suggestion.

 

[Tarheel]

A brief dialogue.

Jill hears a knock at the door and goes to see who it is. She looks through the peephole and sees that it is John. She opens the door a crack. John says, "May I come in?" Jill says, "Please do!"

In the original dialogue John opens the door himself, indicating that he could have let himself in.

 

[Uncanny]

No. For a start, John's line needs to use the correct word order for a question. However, note that BrE doesn't use "Will I ..." for a question. We would say "Shall I come in?" I don't know about AmE. I do know that many people in Ireland would use "Will I come in?"

What about "Will one come in?" or "Am I coming in?" in the sense of "Will I come in?"?

 

[Rover_KE]

Those are not natural.

 

[Uncanny]

What's the difference between "Shall I come in?" and "Do you want me to come in?"?

 

[emsr2d2]

What's the difference between "Shall I come in?" and "Do you want me to come in?"?

The first is a suggestion in the form of a question and the second is a direct question.