Sam Witwer is about to become a werewolf. No, he's not swapping supernatural curses with Sam Huntington on Syfy's Being Human. Instead, he's trading in his vampire fangs for claws and a fur coat to guest-star as a Wesen with a problem on NBC's Grimm tonight.

“I'm playing a werewolf, which is a real shift,” Witwer told Blastr in an exclusive interview. “I've got to tell you, Huntington does it better, as does Silas.” Silas is Silas Weir Mitchell, who plays Monroe on Grimm, a reformed big bad wolf Wesen or a Blutbad.

In the episode, “The Show Must Go On,” Witwer's Grimm character is a Wesen who is “part of this freak show. And because he's forced into this creature for this freak show, he does it so much that he develops this condition. When they turn too much, they start losing control. They start losing who they are. They start losing their identity to the beast, as it were,” he said.

“Basically, there are some murders. And presumably my character is committing them. So that's were we start. This guy is kind of the bad guy,” said Witwer.

“The way that the story is structured, there wasn't a lot of opportunity to delve into this character other than the fact that he's a man in crisis. So we're not going to get a lot of sense of humor from this guy, for example. He is basically his problem right now, and that's basically what we explore in the episode,” he said.

Getting furry wasn't easy

For Witwer, there were some challenges when it came to sinking his teeth into the role.

“I honestly didn't understand how to play it. I can tell you that. It took me a few days to know what I was doing on that show, and they were like, 'Well, what do you mean? You're on Being Human.' And I'm like, 'Dude, this is not what we do. Being Human is hanging out in the kitchen and cracking lines at each other.' On Being Human, I play a heroin addict, basically. He's not really a vampire. He's a heroin addict. So it took me a second to know what was going on. Silas was very helpful,” he said.

“We're all very happy that we got to end it on our terms."

As for his work on Being Human, which ends its run after four seasons on April 7, Witwer said he's proud of the work they've done.

“We're all very happy that we got to end it on our terms rather than having some sort of financial mandate to do something that we wouldn't necessarily believe in,” said Witwer.

“The entire season was to wrap up the series. The fans didn't know that, but we've known this for a year. And so we got a chance to really finish the story as we intended it. That's the most satisfying thing about it, because when you're on a show and you're doing all these stories, especially a show like Being Human where every episode leads to the next episode ... I mean, the whole thing is one big story. We don't do one-offs, you know? When you're doing that, you do wonder, 'Are we going to be able to wrap up this big, epic story in a way that will be correct? In a way that will make the right statements, that will have the right values, that will make this all make sense?' It's with a great amount of relief that I feel like I can report to you that we did that. I think we did,” he said.

Initially the Aiden/Sally romance didn't "make any sense"

One big development this season has been the romance between Witwer's Aidan and Meaghan Rath's Sally.

“The fans have mostly been for that, but then there's a fairly large contingent, like 30 percent, that are against it. And I just wonder by the end of it how much we will have turned them, because we've certainly been having a lot of fun with it. I definitely read on Twitter after the last episode, there were a lot of people chiming in going, 'OK, OK. You got me. I was against it, but now I'm for it.' It's, hopefully, because we've been having fun with it and we've been doing unexpected things,” said Witwer.

The actor admits that when the idea for the romance first came up, he wasn't exactly for the Aiden/Sally of it all either. “I was against it when the writing room would bring it up early on. I was like, 'It doesn't make any sense. Sally's like a little kid and Aidan is this really old guy. They just don't have that in common. She's too naive and Aidan is too jaded.' But as the series went on, Sally grew up a lot and Aidan kind of lightened up. Suddenly these characters were sharing a lot more common ground than they used to. If you watch season three, there's always some sort of weird spark between Aidan and Sally. It didn't necessarily exist in seasons one and two. But in season three there would be scenes that were not written with any kind of romantic slant to them, and then next thing you know, me and Meaghan Rath, there's just a weird vibe. You're like, 'What's going on with these two? What's happening?'

“Between the seasons, Anna Fricke got a hold of me and she's like, 'Okay, we're wrapping up the series this year. What do you think? Do you think it could happen?' And I'm like, 'If you asked me at any other time I'd say no. But after season three, let's do it. Let's go for it. Why not? It might be really fun.' I think the way that they did it was fantastic, to have these alternate-universe episodes where Sally goes off to essentially a different portion of the show where all the characters meet each other in different ways. And fourth-season Sally is a pretty good match for first-season Aidan,” he said.

"I think we did the name proud."

“The good news is that it's a complete story and it will always be on Netflix. It will always be available out there. Between the British series that did 37 episodes and we did 52, there's a lot,” said Witwer.

The British Being Human completed its run last year, meaning that, for the first time since 2008, there will be no series called Being Human on TV after the Syfy version leaves the air in April. The British series was created by Toby Whithouse, and then the story was developed for Syfy by Jeremy Carver and Anna Fricke.

Although the storylines for the shows veered in different directions, the core story remained about a vampire, a ghost and a werewolf who live together as roommates, all the while struggling to keep their humanity.

“There's a lot of stories about them. I think we did the name proud. Thankfully, I'm happy to report, Toby Whithouse seems to agree with me. He wrote me a very nice email recently expressing that he felt we did it right,” said Witwer.

Here's a preview of tonight's Grimm, “The Show Must Go On,” which airs at 9 p.m. on NBC:

Grimm airs on NBC on Fridays at 9 p.m. Being Human airs on Mondays at 9 p.m. on Syfy.

Are you ready to see a whole new side of Sam Witwer?