Josh Allen is calm, cool and collected under pressure ... but he was anything but when he had a ring in his pocket -- admitting he was shaking in his boots when he proposed to his now-fiancée, Hailee Steinfeld.
The engaged couple took part in a Q&A a week after their big news was made public ... peeling back the curtains and letting fans in on their relationship.
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The actress/singer asked the Buffalo Bills quarterback for his take on their "most magical day" as he came off as "so chill" at the time .