For the role of the action hero, they needed an actor who was convincingly well-built.

well-built 是由副词 well（好地、充分地）和形容词 built（体格健美的）组成的复合形容词。built 来自动词 build（建造），在这里引申为"体格构造"的意思。well-built 合起来表示"体格健壮的、身材魁梧的"，形容人身体结实有力。