(CNN) Joaquin Phoenix may have won his first Oscar for his role in "Joker," but the actor used his moment in the spotlight to call attention to serious causes.

Phoenix picked up the award for best actor, becoming the second performer to win an Oscar for playing the comic book villain. In his speech, he delivered a message of unity, saying whatever the cause was close to a person's heart -- be it gender inequality, racism or animal rights -- "we're talking about the fight against injustice."

"We're talking about the fight against the belief that one nation, one people, one race, one gender or one species has the right to dominate, control and use and exploit another with impunity," he said.

He also said he hopes others are granted what he has been previously -- a chance to be better.

"I've been a scoundrel in my life. I've been selfish. I've been cruel at times, hard to work with, and I'm grateful that so many of you in this room have given me a second chance," he said. "And I think that's when we're at our best -- when we support each other. Not when we cancel each other out for past mistakes, but when we help each other to grow, when we educate each other, when we guide each other towards redemption. That is the best of humanity."

He concluded by emotionally quoting a lyric written by his late brother, River Phoenix, when he was 17 years old: "Run into the rescue with love and peace will follow."

The first person to win an Oscar for playing the Joker character was Heath Ledger, who won posthumously in 2009 for his role in "The Dark Knight."

This is the second time two actors have won Oscars for playing the same character.

In 1972, Marlon Brando won a best actor Oscar for playing Vito Coreleone. Two years later, Robert De Niro won best supporting actor for playing a younger version of the same character.

In 2005, both Phoenix and Ledger competed against one another in the best actor category for their roles in "Walk the Line" and "Brokeback Mountain" respectively. They lost that year to the late Philip Seymour Hoffman, who won for "Capote."

The award is Phoenix's first Oscar. He previously was nominated for "The Master," "Walk the Line" and "Gladiator."