I was just re-reading a cute story, about two women, slowly falling in love with each other, when I read: "Will I  top up your glass?"

Normally, this Sentence only makes sense if the speaker doubts whether she would do it.  The way I would say it is "shall I top up your glass?"

It might be clearer if you look at a plain sentence, rather than a question; we can later check out whether changing a statement to a question makes a difference. 

"I shall top up your glass."  This person has decided to top up somebody's glass.

"I will top up your glass."  Here, the speaker will top up the glass, unless there's a problem. 

"You shall top up my glass."  That's an order. 

"You will top up my glass."  That's a prediction.  If the "will" is emphasized, like with italics, it's a threat.  You will top up my glass.  You have resisted me too often! 

Gosh, that took too much out of me... no wonder I was such a mediocre teacher ...

Okay, now ze questions. 

"Shall I top up your glass?"  This is a simple Request for information.

"May I top up your glass?"  This is a polite request for permission.

"Will I top up your glass?"  Hmm, I wonder.  This is complicated; it suggests that glass-topping-up has had overtones of power play. 

There's always the chance that the—increasingly common—use of "will" instead of "shall" is a sort of regional, or generational, dialect element, that was thrown in to emphasize the age difference between the two women.  If so, I'm too much of an outsider to catch it. 

Well, so far this is getting to be a fun Christmas.   Hope you readers are enjoying yourselves!

Kay Kringle