Will Ferrell's son explains 'bizarre' thing that Step Brothers co-star John C. Reilly did during a Christmas dinner at their house

Will Ferrell's son revealed the weirdest thing that's happened at a dinner in their family home, and it's not surprising.

It's all to do with Ferrell's Step Brothers co-star John C. Reilly, and if you've seen the 2008 film, it's very in tune with the character.

The former's eldest son, Magnus, spoke about how he came over for a Christmas dinner and did something that made the 20-year-old class him as 'the craziest person that came over for a family dinner'.

You've probably seen Magnus before, as he was behind his father when Ferrell was bizarrely seen behind the DJ decks at his college son's frat party.

Ferrell and Reilly teamed up for a number of memorable films in the past 20 years, such as Talladega Nights: The Ballad of Ricky Bobby and Holmes & Watson, but Step Brothers is arguably the most iconic.

Whether it be the drum set argument, the creation of all-time tune 'Boats 'N H**s', or the finale at the Catalina Wine Mixer, the title has gone down as one of the great 2000s comedies.

But the actors' relationship goes beyond work, with the two still close friends, as Magnus Ferrell explained what happened at that fateful dinner.

Speaking as a guest on Joe Vulpis' Lightweights Podcast, he recalled: "We were doing this Christmas thing where we have close friends and family (over), John C. Reilly was there and at one point it was like pretty normal, and then at one point, he started singing opera,

"He just performed an opera song, it was the most amazing thing - like it was bizarre but you're like 'wait, this is so funny,'" he revealed while laughing.

Magnus was asked by Vulpis if he was allowed to watch many of Ferrell's films, like Step Brothers, as he grew up.

"Not till I was older," he replied, adding that there are still some comedy classics that he hasn't got around to seeing yet.

He admitted: "I haven't seen Blades of Glory, which I think is kind of a crime.

"Step Brothers, I think, is my favorite one out of these movies, I just think it's funny," he added.

Admitting that he 'needs to catch up', he also admitted that he hasn't seen The Other Guys, which stars Mark Wahlberg, all the way through.

In all fairness, it can be hard to keep up when your dad's a comedy legend.

Speaking about Magnus' Reilly admission, one user commented: "John C Reilly is an unbelievable vocalist!!"

Another pointed out: "People are unaware that John C. Reilly is actually professional singer. Watch Chicago"

A third wrote: "This is so wholesome i love it."