The first thing to come to mind when someone mentions Pacific Rim – and now its sequel, out this Friday – is bombastic cartoon violence between kaiju monsters and giant mecha, writ large in 3D-IMAX glory. In order for any of the movie’s puny human characters to make as big an impression as that scene where a Jaeger uses a cargo ship to beat the snot out of a kaiju, they have to be pretty bombastic and cartoonish too. Enter: Charlie Day as just-shy-of-mad scientist Newt Geiszler.

To be sure, Day has some competition for the ORWAV Scene Stealer crown (a completely real thing). A bushel of colourful performances prop up the pretty dull lead pair of Charlie Hunnam, who can act but is rarely asked to, and Rinko Kikuchi, who can do no wrong but is underused here. Idris Elba puts some welly into his instantly-memetic “cancelling the apocalypse” speech, and pulls off the name Stacker Pentecost shockingly well. Meanwhile, Ron Perlman pulls off some gold-plated shoes, and Burn Gorman affects both a limp and a silly accent, but each is outshined by their most frequent screen partner: Charlie Day.

Charlie Day

Courtesy of: 20th Century Fox Home Entertainment

When Newt first appears alongside partner-in-science Herman Gottlieb (Gorman), the pair’s role seems to be purely comic relief. Newt’s signature rambling is a welcome bit of silliness in a giant-robot movie that occasionally takes itself too seriously. This breathless technobabble will be familiar to anyone who has previously seen Day on his long-running sitcom It’s Always Sunny in Philadelphia. There, Day plays Charlie Kelly, an insane and largely oblivious janitor with a talent for bashing rats. The characters aren’t exactly identical but, given a change of context, Charlie’s mania maps pretty directly onto Newt’s enthusiasm for kaiju-science.

Day was reportedly cast as Newt based on one specific scene from Always Sunny. Pacific Rim director Guillermo del Toro is a fan of the show, and has even made two cameo appearances as Pappy McPoyle. According to Day, the scene that inspired Del Toro to cast him came from the 2012 episode “Charlie Kelly: King of the Rats”. In the episode’s cold open, Charlie emerges from a marathon rat-bashing session looking haunted, and begins to contemplate the relative value of rat and human life:

It’s a rare subdued moment for Charlie, and the gag comes from the contrast between his seriousness, the insanity of the situation, and the rest of the Gang’s apathetic reactions. Day’s ability to take an absurd character, play him seriously, and get laughs clearly recommended him for the part of Newt. You could even connect the two characters further, by way of Charlie wondering “if our lives are really more valuable than theirs, you know what I mean?” While Newt isn’t exactly a kaiju sympathiser, he is driven to understand the creatures beyond how best to bash them with a big stick.

Day is given the chance to really start stealing scenes once Newt breaks out of the comic-relief sideshow and becomes a vital part of the plot. Looking for more information about the kaiju, Newt puts himself in danger, squares off against a shady black-market dealer (Perlman), and narrowly survives a monster attack. Day adapts to his character’s action-movie turn without feeling like a different person – Newt’s nervous, bumbling energy remains a constant, as when Perlman shakes him down for information:

“That’s classified, so I couldn’t tell you, even if I wanted to.”

“But it is pretty cool, so I might tell you.”

“I’m gonna tell you.”

Charlie Day

Courtesy of: Warner Home Video

Whoever ended up playing Newt was going to have to sell a lot of plainly ridiculous dialogue about kaiju brains and drifting and such. Del Toro made the perfect choice casting an actor known for comedy, particularly the clever-stupid absurdity of Always Sunny. The real charm of Day’s performance here is his willingness to commit wholeheartedly to selling this bananas dialogue. In doing so, he becomes more or less the emotional heart of the movie. Newt’s passion for figuring out the kaiju, and his terror when under attack, are as broad as anything else in Pacfic Rim, but Day plays them with the same sincerity that has made Always Sunny’s Charlie weirdly lovable through 12 seasons of reprehensible antics. What is more, Day clearly loves the role and Pacific Rim as a whole, going so far as to pen a few “Dayman”-calibre prospective theme songs for Uprising.

Where most of Pacific Rim’s supporting cast consists of (greatly entertaining) broad caricatures, Day brings some humanity and a little depth to Newt. He is our viewpoint character whenever the dashing lead duo are off-camera, and as we spend time with him he morphs from a two-dimensional comedy scientist to a credible, sympathetic protagonist. Day’s good-natured performance even elevates his mad-science double-act with Herman into a genuinely sweet friendship – or more. Plenty of fans, Day included, have read the paring as romantic (more so than Hunnam and Kikuchi’s chaste partnership). Will Uprising expand on this aspect of their characters? Unfortunately, the odds are against it. Viewers eager for more may have to settle for Gorman’s winking cameo as a scientist on Always Sunny:

Charlie Day reprises his role as Newt in Pacific Rim Uprising, directed by Steven S DeKnight, out in UK cinemas Friday 23 March.