What does なり切ってしまう mean?

Question

I've been reading a book in Japanese in order to learn new kanji, words, etc., but I've come across something I can't seem to translate. The sentence is as follows.

老人にも若者にも、富豪にも乞食にも、学者にも無頼漢にも、イヤ女にさえも、全くその人になり切ってしまうことができるといいます。

I'm familiar with all the words and grammar within the sentence, and, based on context clues, I think I have a rough understanding of what this sentence means. But I'm a bit uncertain of what なり切ってしまう actually means, as I don't think we're talking about the verb 'to cut'. It's my understanding that 切る has a good handful of other meanings, so I thought it must be one of those which I'm unfamiliar with.

And, to give a little bit of context in case that helps, this is from Edogawa Ranpo's novel「怪人二十面相」and the sentence above is describing the thief's ability to disguise himself.

Answer 1

「富豪{ふごう}にも乞食{こじき}にも、学者{がくしゃ}にも無頼漢{ぶらいかん}にも、イヤ女にさえも、全{まった}くその人になり切{き}ってしまうことができるといいます。」

「なり切る」 means "to really get into the role", "to act a role perfectly", etc.

「なる」 in this expression means "to become ~~" in the sense of "to play the role of ~~"

「切る」 here means "to do something completely or to the end".

"From tycoon to beggar, scholar to scoundrel, and even a woman -- (this man) is said to be capable of acting any kind of role just perfectly."