NOTE:  Compare these qualities with those of Cancer Robin Williams, and just substitute the fixed Scorpio with the dynamic fired fire of Leo. 

Sun: Leo
As one of the consummate actors of his generation, Robert De Niro shows the flare and drama you'd expect from a Leo Sun-- but in his roles, he's often plays characters with a disturbing edge.  Early on, this left some of us guessing that this star of "Taxi Driver" and "Deer Hunter" was "heavy Scorpio".  As his chart revealed, he has no Scorpio (save for a empty fifth house).  However, he does have Pluto, Jupiter and his North Node all in Leo, progressively forming a loose conjunction with his Leo Sun.  This powerful grouping gives him a concentration of fixed energy.  Furthermore, a Mars in Taurus (square his Sun) makes his physical movements firm and fixed.  Add a water ascendant and moon to this, and our guesses become "archetypally" correct.
Ascendant: Cancer 
Regardless of how bizarre the role, De Niro often shows "a softer side", i.e., his Cancer ascendant. In the film "King of Comedy" captured the dramatic extremes of a "lion/crab" combination. There, Robert played an obsessed comic who kidnaps a famous talk show host. Throughout the movie, this loud mouthed "King" would throw his solar tantrums, but sporadically gushes of emotions would swell to the surface-- to round his facial features and mask his fires. In these quiet (non-ego) moments, he would hover over his victim like a mothering hen. Cancer's protective presence was also noticeable in his relationship with Jody Foster in "Taxi Driver".
Moon: Pisces 
In our introduction, we mentioned how the placement of the ascendant ruler gives a way for individuals to "embellish on their public mask".  De Niro's Cancer ascendant is ruled by the Moon and his Moon is in Pisces.  Consequently, he has an ability to miraculously alter his appearance.  In his role in "Raging Bull", he set a new standard in film performance by totally changing his physical being-- turning from lean to muscular, then pudgy to overweight.  Likely, his Piscean emotional temperament helped him make the sacrifices for this very demanding role. 

In the film "Awakenings", Robert portrayed one of his quieter and finest roles as a patient emerging from Neptune's net to regain "conscious expression".  Unhampered by ego or physical desire, we saw a person who expressed his lunar feelings by openly reacting to the world with bewilderment, amazement and joy! 

Note how in his reactions, De Niro's rounded water features become distorted.  The resultant skewed facial lines give him a dreamy, disarming appearance.  Positively, this lunar placement adds a compassionate dimension to his normally self centered Sun energies.