It wasn't so much that their characters used each other to grow, its that their characters wanted to be what the other one had. Russel Crowe wanted to be warmer and have a family and a day job, where Ryan's character wanted to be tough and harder. They were able to learn from each other and grow in that sense, and kt worked GREAT. Not every film has good character arcs that allow those characters to work so well togetber. In this case, it has nothing to do with chemistry. It was the writing. The chemistry of the actors sold us the story because these two are gems, but I think you dont understand chemistry vs writing of character arcs. Understanding character abilities and flaws and what role they fit, like antagonist, foil, etc. There are even great films where the acting and writing was superb even though their wasnt great chemistry with the cast. Vice-versa too - arguably bad films that only worked because the actors had such amazing chemistry you could overlook the roughness.

my man, you can't just say they have good chemistry because they have good chemistry, and the movie works because they have good chemistry, for seven minutes. Explain, show scenes, go into detail, don't take seven minutes to deliver three sentences worth of information.