The 33-year-old actress - who was dating her former co-star when he died from an accidental overdose in 2013 - admitted she never used to feel so touched by particular moments from the musical TV show but now it really gets to her when she sees her ex-lover's alter ego, Finn Hudson, showing his ''heart''.
Lea joined 'Glee's Kevin McHale and Jenna Ushkowitz's new podcast, on which they will be recapping old episodes of the series - to talk about the programme's pilot episode, which she rewatched before recording the discussion.
She said: ''The moments that made me so emotional were not anything that ever has made me emotional before about the show.
''One of them was when [Monteith] took [McHale] out of that port-a-potty. There were so many moments that you see the whole arc of a character for a whole season in one moment.
''Him taking [him] out of that bathroom, and you see Finn's heart. Oh my God.''
Kevin - who played Artie Abrams - agreed the scene had been one of his ''most memorable parts'' of filming the first ever episode.
Cory's mother, Ann McGregor, previously revealed Lea broke the news of Cory's tragic death to her.
Ann said she'd never be able to forget the moment she found out the news as the actress rang her up ''screaming'' down the phone.
She recalled: ''I got a call from Lea and she was screaming on the phone. She was yelling, 'Is it true, is it true about Cory?' and I said, 'What about Cory?' I had heard nothing. And then police knocked on my front door. I went into a state of numbness. I just shut off all emotions, and I was numb.''
Although Ann eventually came to terms with the loss of her son - who died at the age of just 31 - there were, initially, times when she'd convince herself Cory would call her ''soon''.
She explained: ''I knew it was real, but there were times when I could lie to myself and say, 'He's in Los Angeles, he'll call me soon.' I'm still always hanging on edge.
''When you lose someone who is such a big part of you, you lose all purpose for living.''
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