The second half of the hit show's fifth season premieres May 28 on Netflix.
"It has been a long time in the planning. We have done a lot of music in the show and done some dancing in the show. People were always like, 'Are you going to do a musical episode?'" Ellis, who stars in the titular role, said. "... We always wanted to find a way to do it where it wasn't just a musical episode. There needed to be reason for the music and dancing to happen."
That reason, Ellis said, "is God indeed," with whom Lucifer is having "huge issues, huge unresolved issues" when the episode kicks off.
"Basically Dad comes to visit because his children are not getting along," Ellis explained. "Basically God comes down to sort it out and decides he is going to stay for a while... to follow his son around. It's quite fun. He just kind of goes, 'I'm going to make them sing and dance right now.' Lucifer is like, 'Why do you keep doing this? Just leave me alone, Dad.' That is sort of the progression throughout this episode."
The pair even have "a really lovely moment" while singing a duet, Ellis said. While the actor wouldn't reveal what song Lucifer and God, who is played by Dennis Haysbert, sing, he did say that it's one that "a lot of people know."
Their duet and other interactions, Ellis teased, lead to a "pivotal moment" in their relationship.
"Lucifer has issues with Dad and Dad is sort of pretending that nothing has ever been wrong, which of course winds Lucifer up," he said. "He spends most of his time just [trying] to keep away from him, and then he has sort of a revelatory moment in this episode where he is like, 'Maybe for the sake of other things in my life I should try getting along with my Dad.'"
Lucifer's "breakthrough with Dad" should make fans hopeful about the future of his relationship with Chloe (German), Ellis said, even amid "a bit of a sticky point" in their relationship.
"Lucifer sort of has a revelation that he thinks that he is not capable of love," Ellis said. "... He hasn't said the magic words to her."
"Now Chloe is kind of in a place where she is a little past the heartbreak of that, but now she is going, 'No, I don't believe that anyone is incapable of love, and especially not you since I know you well,'" German added. "... She believes that he is absolutely capable of love, and is just trying her very best to maybe convince him a bit without pressuring him."
Chloe's approach, Ellis believes, will have a positive outcome for Deckerstar.
"Finding the difference between feeling something and saying something about that feeling is sort of a very difficult thing for him. So Chloe is being very patient with him at the moment [by saying,] 'I am going to give you the time to just come to that realization yourself,'" he said.
"We are not great at finding our love, but we are very great at communicating," German added. "I am always asking and always giving an honest answer. They aren't really hiding anything from each other. The love is there. The word might not be said, but the communication is there and that gives people hope."
Both Ellis and German believe that God approves of Chloe, especially after their first meeting, which takes place at a crime scene that turns into a flash mob during the episode.
"I am unhappy with God a little bit," German said of her character's feelings in the moment. "With all of the frustrations that Chloe has with Lucifer -- not being able to be loved, kind of knowing about his background, and feeling a bit that Lucifer was abandoned and rejected and set down -- in this moment that takes place on the field when Chloe meets God, she kind of lets loose on God and tells him that she does not think he is a good parent and sort of gives him her two cents."
"Lucifer has never loved her more until that moment," Ellis said. "I think she is going to do what everyone does, 'Oh my God. It's God. I love you. You are brilliant,' and she does the exact opposite."
As for how God responds to Chloe's criticism, German said he does so "in the most beautiful, most classy, God way."
On top of Lucifer's big moments with both God and Chloe, the episode will also include a very special guest star, Debbie Gibson, who will play the mom of a football player at the high school where the murder takes place.
"Oh my God, it was amazing actually," Ellis said. "... She came and we did a song together... Obviously she is an amazing singer, but she was the loveliest. Not only that, she acts in it as well. She's a great actress."
It's Lucifer's opening number, though, that Ellis considers to be his "favorite song that I have ever sung on the show." That song, Ellis said, features one of a few notable lyric changes in the episode.
"In the first song that Lucifer sings, he drops a very important lyric from the first song because this song is a bit personal to him," Ellis teased.
Overall, Ellis said, the episode is "without a doubt" the "biggest" one they've done.
"I am having the time of my life. I cannot wait for people to see this episode," he said. "... I hope that they love it."
"People that love musical theater and musicals are going to absolutely adore it," German added, before sharing why one of Lucifer's numbers made her emotional.
"I was in my trailer just crying at how beautiful the moment is, just knowing the moment in the story, it brought tears to my eyes," she said. "It is so absolutely touching and gorgeous and sung so well... You can feel what he is feeling through the singing. It's beautiful."
For Ellis, that moment, and the episode as a whole, is everything he hoped it'd be and more.
"I am super proud of the entire episode and how it has turned out. This is something I have wanted to do on the show for a while. We needed to find the right moment, and this is totally the right moment," Ellis said. "I am just delighted with how it has turned out."
Lucifer 5B premieres May 28 on Netflix.