"Every moment of this is pure agony."
When Anderson started courting Hackman for the role, he sent the actor some preliminary artwork to help convey the tone of the film, which Hackman never returned, possibly because he tossed it straight into the trash. A portion of his s**t-boiling rage was stirred by the fact that he resents being offered roles that were written with him in mind, which he told Anderson in a face-to-face meeting before Anderson had even started writing The Royal Tenenbaums. The other reason was money -- The Royal Tenenbaums didn't have much of a budget, and Hackman's character is in nearly every scene, which means he'd essentially have to work every single day of filming for a much smaller salary than he was used to.
The scene where he bet on a dogfight wasn't actually in the script.
Hackman eventually let his agent talk him into accepting the role, but not before making a solemn vow to be the biggest son of a b***h he could during filming. Making sure everyone working on The Royal Tenenbaums knew he'd literally rather be beaten to death with coat hangers than be in their f*****g movie was just below "turn in a good performance" on his list of priorities. According to the cast and crew, Hackman became increasingly intolerable as filming went on, venomously snapping at people for the slightest transgression and generally making Anderson's life miserable. He treated Anderson like his no-good, hipster nephew, telling him to "pull up his trousers" and straight up calling him a c**t in front of the crew.