Jack transformed himself for Ennis in a
Zelig-like way, hiding his gayness and his loquaciousness.
def = a chameleon-like person, named after Leonard Zelig, the hero of the 1983 movie "Zelig" by Woody Allen.
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=clarification= I believe that Jack was most comfortable with himself while around Ennis and that Ennis got to see the real Jack. However, Ang Lee has been referred to as a
Zelig-like director:
"That most chameleon-like of directors, Ang Lee, pulls off yet another surprising left turn in 'Brokeback Mountain.'" -- Todd McCarthy, Variety.com [link] Fran