DeNiro has perhaps played a little too often for his own good, and then they've gotten DeNiro himself to play it. He's wonderfully funny, narrowing his eyes with crazed malevolence, then embracing his daughter like the loving father he clearly is. Stiller, too, is first-rate. Your basic pretty-boy young leading man would have been blown off the screen by DeNiro, but something about Stiller gives him enough heft to hold his own. What really matters in a movie like this is that everyone be on the same page.
Tone is critical, and here they get it right.
Polo is an effective mix of loving girlfriend and oblivious daughter, and has good chemistry with Stiller as well. Blythe Danner, playing Pam's mom, is to some degree spoofing herself just as DeNiro is, working off of the fact that she must have played more gentle-hearted mothers in movies of this kind than almost anyone. Her lines aren't as good as DeNiro's, but she's lovely and sweet nevertheless. He has such absolute sincerity that he genuinely appears to be trying to help, even as he substantially and constantly deepens Greg's misery. I think the movie loses its way a little in the last section, reaching for a warm and fuzzy feeling that perhaps doesn't fit.