While Costner had about six years of big screen experience under his belt when he made the Brian De Palma movie, by that time Robert De Niro had already won two Oscars (for The Godfather: Part II and Raging Bull), and had also been nominated for two others (for The Deer Hunter and Taxi Driver).
Playing Al Capone in The Untouchables is one of Robert De Niro’s most famous performances for a reason – as he went full bombastic with the famed Chicago mobster – but one could see another reason why Kevin Costner might have been a bit intimidated sharing the screen with his co-star.
Regardless of the character being played, that presence alone would put anybody a bit off kilter.