An Insert Song is a song with lyrics that is inserted during a certain scene of a show or series. Usually, this is done to give a scene more emotional impact by using a song with lyrics which relate to the scene in question. Sometimes though, especially if it's a music-oriented show, the creators will have the characters play a song in-story.
What sets the in-story version apart from a regular Musical Episode is that there has to be something important happening in-story as justification for why someone is singing, and that the entire cast doesn't have to be involved.
This is very common in anime, as detailed in Anime Theme Song, since most voice actors/actresses also happen to be popular singers.
Note that the opening theme only counts an example if it's used in the middle of the show, when something is actually happening. And the ending theme doesn't count as an example if it starts during a scene which segues into the credits.
Compare Anime Theme Song (the opening theme, which can be used as this sometimes), Image Song (can overlap with this if the song in question is brought into the show), Leitmotif (when a particular piece of music is tied to a certain character), and Theme Music Power-Up (when epic BGM is playing while a character is kicking ass).
- Angel Beats! has several in-universe due to the Girls Dead Monster subplot, as well as infamous Tearjerker "My Most Precious Treasure".
- In episode 20 of Fate/stay night [Unlimited Blade Works], the song "Last Stardust" plays when Shirou finds the resolve to challenge Archer's beliefs.
- Haruhi Suzumiya has a couple:
- In the finale to Part 2 of Jojos Bizarre Adventure, the theme from the first opening plays again as Joseph finally defeats the Big Bad once and for all.
- In episode 19 of Demon Slayer: Kimetsu no Yaiba, "Kamado Tanjiro no Uta" plays when Tanjiro experiences a flashback during his battle against the arc villain Rui.
- Being a show about girls forming a music club, K-On! has quite a few, such as "Fuwa Fuwa Time".
- The Lyrical Nanoha series has several:
- Mekakucity Actors: Given that it's an anime based on a manga and light novel based on a set of songs, this was bound to happen: (Warning, spoilers)
- "Moonlight" (月光) in Season 2 Episode 16 of Assassination Classroom.
- In Season 4, Episode 13 of My Hero Academia, "Might+U" plays when Deku saves Eri from Overhaul, leading to the climactic battle foretold by Nighteye.
- The "Row! Row! Fight the Powah!" rap is featured in a few songs, most notably "Libera Me From Hell".
- Tengen Toppa Gurren Lagann has several.
- Kira-Kira, being a VN about forming a rock band, has gems such as "Let's Jump", "Kimi no Motoe", and a rock version of their school theme, "O. H. B. I.", all of them performed during their concerts.
- Muv-Luv Alternative has Takeru's Image Song, "Wings", play at the end of the PTSD Arc, when he finally finds his resolve to save the world. "Carry On", another insert, plays at the start of the last arc. The all-ages edition also has the opening play when the last arc is well underway.
- Averted in Halo 2. In-game, we hear the instrumental version of "Blow Me Away" by Breaking Benjamin. The original is featured in the game's soundtrack.
- The Forza Horizon has specific licensed songs play for each "boss" event and Bucket List Challenge, many of which are specifically chosen to tie into the themes of the event. Notable examples include "Train in Vain" by The Clash playing during an event where the player races against a train in 2, and "The River" by Ladyhawke when racing a fleet of speedboats downstream in 3.
- Klonoa 2: Lunatea's Veil has one during the "snowboarding" (or something like it) stage, which is sung in the made up language of the series.
- The Shoji Meguro-composed Persona series has a lot of vocal themes, but uses them in an inversion of the standard expectations - vocal themes tend to play at more trivial moments and the instrumental themes are often reserved for more dramatic, important sequences.
- The standard battle themes in 3, "Mass Destruction", 4, "Reach Out to the Truth", and 5, "Last Surprise", have vocals. Boss themes are entirely instrumental. Persona Q, which wasn't composed entirely by Meguro, has vocal themes for both regular battles ("Light the Fire Up in the Night") and bosses ("Laser Beam").
- For the majority of 3 and 4, the various exploration themes are vocal. During the final month of each game, wherein things begin look more grim and the story is reaching its conclusion, the background music is replaced with usually-somber instrumentals.
- There are a few cases where the "used for dramatic effect" intention is played straight:
- In the finale of Persona 3, a heavy hip-hop remix of the game's main theme, "Burn My Dread", first heard in the opening, plays against the scripted, post-final boss sequence.
- The dungeon themes for Persona 4 and 5 are all instrumental - save for a single dungeon near the end of the game when the stakes have suddenly gotten dangerously high. In 4, the vocal theme is for the dungeon Heaven, which the party goes through when the protagonist's seven year-old cousin is kidnapped. For 5, it's Niijima's Casino, where the storyline comes back around full circle and everything is clearing up just before the game's final act.
- In 5, the instrumental version of "Life Will Change" replaces the standard dungeon themes when the Phantom Thieves are on their way to finish the dungeon. Near the end of the game, it upgrades to the vocal version, first heard in the prologue level.
- A single boss theme in 5 has vocals rather than being purely instrumental: "Rivers in the Desert", the theme for Shadow Shido, the big bad who ruined the protagonist's life at the beginning of the story.
- Saints Row became fond of this beginning with the third installment:
- Villain Song "POWER" by Kanye West, which more-or-less serves as the game's Real Song Theme Tune, plays in the background of the mission "Party Time". As said mission consists of the Saints crashing a penthouse party to steal it and use it as a base of operations, it's certainly a fitting jam.
- Used for hysterical effect during "Murderbrawl XXXI", where the inspiring "You're the Best" by Joe Esposito plays as the Boss uses a chainsaw to slaughter a gang of luchadores during a live wrestling broadcast.
- "Holding Out For a Hero" by Bonnie Tyler plays during the mission "Three Way", as the Boss has to decide between saving several of their closest allies or leaving them to die and opting to go for revenge on the main Big Bad.
- Super Mario Odyssey is the first Mario game in history to have vocal songs of any kind. In this game, there are two: "Jump Up, Super Star!" plays during the New Donk City festival, while "Break Free (Lead the Way)" plays at the end of the final level, when Mario and Princess Peach escape the underground lair in the wedding hall on the moon.
- RWBY: Most of the songs in RWBY are have their lyrics played in the show, especially during fight scenes.