Need a handsome hero with irreverent charm and a dry sense of humor? Ryan Reynolds is very obviously your guy.

Need to cast a tiny yellow monster who doesn’t typically speak English?

Yeah, Reynolds is a fit for that, too.

At least that’s how the filmmakers behind “Pokemon: Detective Pikachu” (in theaters Friday) felt when they were seeking the voice of their titular character. Reynolds plays Pikachu, one of the many colorful Pokemon in the human/monster town of Ryme City, who teams up with human friend Tim (Justice Smith) to unlock the mystery of the disappearance of Tim’s dad, Pikachu's private-eye partner.

On its surface, casting Reynolds seems a strange decision. In practice, it’s even more bizarre because Reynolds, as Pikachu, is really just riffing and acting like himself. The actor was encouraged to improvise in the role.

And, no, Reynolds, 42, didn't study the behaviors of big-eared critters and/or the facial expressions of anime characters with red cheeks and electric tails to play the part.

“I don’t know if you’ve taken a gander at my resume. I’m never that prepared,” Reynolds jokes. “(Pikachu) is an adorable, mythical pocket monster. It’s not like I’m going to lose 180 pounds" to play him.

So Reynolds, as is, embodies the cuter-than-a-kitten Pikachu. And that’s exactly how director Rob Letterman wanted it.

Letterman approached Reynolds to be his film’s star after auditioning the actor without Reynolds knowing it: He animated Pikachu to a clip of Reynolds' audio from 2011 comedy "The Change-Up" to find out what it would be like to pair the impossibly adorable Pikachu with the actor’s signature warm-yet-sarcastic voice.

“We had to get a big personality out of an adorable character,” the director says. It was obvious to him upon seeing the animated clip that Reynolds was the perfect man for the job.

The result onscreen is hilarious, especially when the big-eyed Pikachu – who before this movie and its companion video game only cooed the word "Pika" over and over again – utters lines like, “I push people away and hate them for leaving” (a Reynolds ad-lib). Or, when Pikachu invites Tim to sleep at his apartment, mock-demures: “I never do this. I’m not that kind of Pokemon.”

“Detective Pikachu really is Ryan,” says Letterman, just with expressive, fox-like ears and a teensy brown nose.

And it's not just the voice that’s Reynolds’ – it’s the facial expressions, too. The actor wore a motion-capture facial rig for the first time for “Detective Pikachu,” and performed the movie’s entire script on a recording room stage with cameras capturing “micro-facial expressions,” Reynolds says.

“They scale (my face) down and put it on a character who’s much more rotund and small and round,” says Reynolds, who says seeing himself projected onto Pikachu is truly bizarre. "I notice (the resemblance) more than other people.”

But the actor’s kids are beginning to catch on: “My older daughter has said about Pikachu, ‘You sound exactly like that guy.’ ” (Reynolds and wife Blake Lively revealed on the "Pikachu" premiere carpet that they're expecting a third baby.)

His daughters, James, 4, and Inez, 2, associate their father more with his red-suited, knife-fighting superhero character Deadpool, because they visited Dad on that set more. For “Pikachu,” Reynolds was only on the set for about a week.

When Reynolds wasn’t there, his co-star Smith acted opposite a tennis ball or puppet. Letterman combined Reynolds' human-sized movements and facial capture with a computer-generated Pikachu, and edited them into scenes with Smith.

There was plenty to work with.

"I threw everything up against the wall," says Reynolds about his alternate lines of dialogue. "There was some stuff in there where I went a little blue. I forget what, exactly, and it doesn't even matter because It's unprintable. There's probably a gag reel somewhere that would take down the entire industry, or certainly the studio."