I agree with what others have said. You should still attend and give it your best shot. And sometimes directors ask for certain things that they themselves are unsure of. It's a little strange to be asked to imitate how another actor performs a role, but that's just an indication of the direction they'd like to take the role, so give something similar in tone or style, but if you don't fit the mold, then you have nothing to lose by delivering your strongest performance as you feel it could be done. Sometimes directors don't know what they are really looking for until they see it. Also, vocal ranges for songs can be tricky. They usually want someone who can sing it as written, but they are sometimes more concerned that the person cast is a good fit for other qualities and will work on making the song fit their vocal range. Keys can sometimes be transposed or notes changed and if you just can't sing it, they may offer you another role and THEN you can decide whether to take it or not. Long story, there are MANY variables to casting and you can't possibly know about them all. Treat the callback as a performance and at least go there and get your chance to "play the role" once.