Well, I think when you're captivated by a particular artist's interpretation of a well-known song, it's valid and admirable to try to "copy" it ~ especially when the version in question is a departure from earlier, more familiar renditions. (e.g., Jimi's "Watchtower," Joe Cocker's "Little Help from My Friends," to cite a couple of very well-known rock/folk-rock examples).

The thing to keep in mind is that you CAN'T absolutely duplicate that other person, you can only be you. So, as you continue to practice and perform the song over and over, you will be adding your own wrinkles, unconsciously if not consciously.

Eventually, your version should develop and emerge, and it should have its own quality and appeal. Even if you're technically less sophisticated than the artist you're emulating, something about your performance will be unique, and therefore (if you have any musical talent at all) worthwhile.

Mention of Eva Cassidy's "Over the Rainbow" reminds me that I've recently been hearing another very original reinterpretation of the same song, sung and played on the ukelele by a male Hawaiian artist. Very nice, and very different. Anyone know what I'm talking about?