With the premiere of the musical movie less than a month away, the "Wicked" flick has given another glimpse behind the curtain of the making of this saga ... including footage of Ariana and Cynthia belting their hearts out on set.

And, before you ask, there isn't any lip-syncing going on ... as both the stars and the film's director, Jon M. Chu, confirmed in the featurette that the powerhouse belters sang live during production.

Check it out ... Cynthia, who plays Elphaba (AKA the Wicked Witch of the West) in the upcoming movie, can be heard doing vocal scales before tackling the song's most famous anthem, "Defying Gravity" ... bringing several crew members to tears.

Similarly, Ariana -- who is playing Glinda the Good Witch -- can be seen casually doing a series of operatic runs while waiting in between takes ... later hitting an impressively high note for her rendition of "No One Mourns the Wicked."

There's even a tidbit of the pair singing a part of "Defying Gravity" together with no backing music ... where the actresses sound as good, if not better, than the Broadway originals. Sorry, Idina Menzel and Kristin Chenoweth.

According to the director, it was originally unclear how much live singing they'd do on the set ... with the common practice being recording the tracks beforehand and having the performers lip-sync along while filming.

Yet, as Jon emphasized, Cynthia and Ariana are some of the "best singers in the world" ... and so they chose to sing live for "every scene."

This decision is certainly a bold one, especially since this same tactic didn't fare as well for the cast of 2012's movie adaptation of "Les Misérables" ... which saw Russell Crowe croaking through his musical numbers.