The actor turned down a $250 million role in the movie.
The 52-year-old actor was offered the lead role in James Cameron's 2009 film Avatar, where he would have starred alongside Saldana and would have earned around $250 million. The role instead went to Sam Worthington, who played Jake Sully, while Saldana portrayed Neytiri. The big-budget movie follows a paraplegic Marine (Worthington) dispatched to the moon Pandora, where Neytiri resides, for a unique mission. He soon becomes torn between following his orders and protecting the world he feels is his home now.
"It’s the dumbest thing an actor ever did in the history of acting," Damon told ET's Rachel Smith of turning down the role.
Saldana chimed in, "I don’t think your career suffered because of it, trust me."
"Do you know what kind of movie that would’ve been if I would had been in it?" Damon quipped before praising Saldana for being in several films that have made over $2 billion, including Avatar.
The 44-year-old actress joked, "I didn't plan it, trust me. I've just always felt blessed that I was picked, that I worked really hard and auditioned. I'm not Matt Damon. I don't get to turn down Avatars."
Damon noted, "I've probably done, like, 50 movies. I've never been in a movie that made $1 billion."
ET was on set with Saldana and Damon for their Stella Artois' Let's Do Dinner campaign, which celebrates authentic connections around a dinner table, with a portion of the proceeds benefiting Water.org, a charity which Damon co-founded that works to bring clean water to those who need it.
"This partnership has been so great for Water.org that I named one of my kids Stella," quipped Damon, whose 12-year-old daughter is in fact named Stella.
In all seriousness, the actor noted, "Stella and Water.org have partnered for the last eight years doing these [spots] and they have just been amazing partners for us. We've been able through this partnership to reach four-and-a-half million people with clean water in the developing world. So it’s been a really wonderful partnership for us."
Damon is particularly impressed with this year's idea: a dinner party. "You couldn’t do dinner without safe, clean water. So it’s just kind of a reminder that some of these things that I think we take for granted, some of the things that are so important for all of us," he explained. "So if you can engage with an issue like this, you’re hopefully helping to provide somebody in a different part of the world with the opportunity to have to take a break and focus on the important things in life rather than trying to find clean, safe water for their family."
Saldana added, "It’s always hard because we're living in an over everything kind of world. Where you’re over worked, you’re over tired, you’re over committed, but family time and bonding is really, really important. I feel like you do have to make time for it. It takes a great deal of work, but it’s so rewarding."
On top of raising awareness and funds for clean water, the Let's Do Dinner campaign also gives fans a chance to win a dinner in New York City with a star-studded guest list.
For more with Damon and Saldana, check out the links below.