The now iconic Indiana Jones costume confused Harrison Ford to the point where he initially questioned many aspects of it. In an interview with GQ, he had this to say:
"It was presented to me as an aspect of character in the first film. My questions about it were many. Why am I wearing a leather jacket in the jungle? Isn't it hot here? Why am I carrying a whip? What am I going to do with a f-king whip? I'm going to whip people?"
He also said that he attached his hat with staples so that it wouldn't fall off during the action sequences, and that he still has scars from the staples. Steven Spielberg and George Lucas explained the choice of costume as reflecting "the movies of the time", an explanation Ford did not question further. Considering how associated with the character the outfit is today, everything turned out very well in the end.
Thanks GeekTyrant