Riggs and Murtaugh are the quintessential cop duo, and Lethal Weapon established the template for many buddy cop movies to follow. At the start of Lethal Weapon, Riggs is the cop who doesn't play by the rules, and the recent death of his wife has left him reckless and suicidal. Whereas Murtaugh is more by-the-book in his approach to his work.
Of the two, Riggs undergoes more personal growth. The criminal case they investigate is personal to Murtaugh, as they uncover a heroin-smuggling ring made up of Vietnam War vets, one of whom served with Murtaugh. Riggs starts off like his usual reckless self, and he and Murtaugh don't get along too well as a result. But over the course of the movie, as the drug smugglers threaten Murtaugh's family multiple times, Riggs does whatever he can to help his partner. The experience bonds them as friends, and that's how Riggs begins to process the grief over his wife's death. At the end of Lethal Weapon, Riggs gives Murtaugh the bullet he'd saved for his suicide but no longer needs. He's grown as a person and his healing has begun.
And in Lethal Weapon 2, it was Murtaugh's turn to evolve.
