Marini, who plays Luc, told Zap2It that he did not know how the performance would turn out when he first heard about it.
"It's going to be that song, 'Baby I Need Your Lovin'," he revealed. "But it's not just going to be singing, it's going to be a massive performance. I'm like, 'Oh my gosh - I have to bring Dancing With The Stars plus I really have to sing?'."
Marini continued: "I went all in. I was not shy by any means. I just went for it. Everyone was impressed, so hopefully it comes out good. There's no tricks. We didn't record anything anywhere. I had my little boom and I was singing and performing."
Marini also joked that Balthazar Getty, who plays Tommy, will have some "breakdancing moves" in the finale and explained that the episode is "loaded" with music.
"Brothers & Sisters doesn't usually deal with music that much," he said. "Pretty much the entire budget was music. It's so good. The audience is going to love it."
ABC recently denied rumours that it has already decided to axe Brothers & Sisters after cast member Rachel Griffiths joked that she will soon be "unemployed". Marini has said that he would like the show to continue for another season.
Brothers & Sisters airs on Sundays at 10/9c on ABC in the US and on Thursdays at 10pm on More4 in the UK.