Lynch told TV Guide that viewers will finally discover why Sue has such a vendetta against the arts.
Executive producer Ian Brennan explained that "Sue at one point had Broadway dreams".
Meanwhile, Lynch revealed that the flashback will see a teenage Sue being criticised after performing 'Oklahoma!'
"Her little heart was broken," Lynch said. "Will tells Sue, 'Poor little Susie Sylvester was told she wasn't good, and now she's got to punish the world'. Like a lot of angry people out there, Sue's a wannabe."
Lynch also praised Colby Minifie, who is playing the teenage version of Sue.
"I think they did a terrific job casting," she said. "She has a great voice and energy."
Glee returns with a new episode tonight at 8/7c on Fox in the US and on Thursday at 9pm on Sky1 in the UK.
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