"I get hate tweets and stuff like, 'You're not Julie Andrews!'" she tells Entertainment Weekly. "I know I'm not Julie. Nobody is and I would never pretend that I was ... I know my place."
Underwood, 30, will take on the famous role of Maria in the NBC special opposite Stephen Moyer, who plays Capt. von Trapp. Audra McDonald, Christian Borle and Laura Benanti will also star in the three-hour televised event.
For her part, Andrews has voiced her support for the project several times, most recently in an interview with Larry King. "I wish it well, that's for sure," she said. "A live broadcast, poor lady!"
The Sound of Music Live airs Thursday, Dec. 5 at 8/7c on NBC.