It's all about certainty.
Let's say that Anne says this to you because Jane is getting ready (to go on stage), and Jane, Anne, and you are all in the same room, Anne would naturally say this:
a) You can't interrupt her now. She is getting ready to go on stage, isn't she?
Now, let's assume that Anne and you are in a hallway near Jane's room. Both of you can't see what Jane is doing at this moment. Anne has nothing to do with Jane's show, but she might know that Jane is probably going to be on stage soon, so logically Jane is probably getting herself ready. Anne might say this:
b) You can't interrupt her now. She might be getting ready to go on stage, mightn't she?
Now, let's assume that Anne, still in the hallway with you, is rather sure that Jane must be getting herself ready at this moment, because she is Jane's manager! However, she can't be 100% sure, because she can't see what Jane is doing right now. Anne would naturally say this:
c) You can't interrupt her now. She will be getting ready to go on stage, won't she?
Here is a small thing. If Anne is absolutely sure that Jane's getting herself ready, she can simply say "is getting"--as in sentence (a)--without having to actually see what Jane is doing.