Things don't go exactly as planned, and it sets the siblings out on an adventure where they discover a lot about themselves, and one another.
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The two actors have been friends for several years. They first connected in the "Marvel Universe", as Spider-Man and Star-Lord. So the chance to be "brothers" on-screen was an easy task.
"When I met Tom about four or five years ago, he was kind of coming into the Marvel Cinematic Universe," said Pratt. "I felt myself in this position to finally be able to extend the same kind of like loving support and voice that I know meant so much to me, hearing it from my brother, and my sister when I was coming up. I would say it to him, things like 'Remember who you are' and 'At the end of the day, you're going to really kick yourself if you don't remain nice' and you know, 'Save your money' and 'Don't take any wooden nickels' and, you know, 'Remain smart and kind' and all that stuff."
The brotherly love extended in both directions.
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"Chris is kinda like my big brother in lots of ways," said Holland. "You're kinda like Barley, happy go lucky, energetic, very optimistic, funny, exciting. And an idiot."
"Yeah, it's true!" laughed Pratt. "All heart, no brain. Wait a minute!"
"Onward" is in theaters Friday, March 6.