[Update: Nathan Fillion has denied this rumor about his Guardians cameo.]

There are few geek-favorite actors sure to cause more online discussion than Nathan Fillion (Firefly, Castle), so when reports surfaced that he would be appearing in James Gunn’s Guardians of the Galaxy, speculation spun out of control. Would he be stepping into the role of Nova, or would his infamous space cowboy ride make a cameo in Marvel’s Cosmic Universe? Gunn shot down the idea of Fillion playing Marvel’s most well-known space cop, but a new rumor reveals that Fillion’s role will still be sure to send die-hard comic fans into the stratosphere.

Since Fillion is both a close friend and regular cast member in both Joss Whedon and James Gunn’s film projects, it was safe to assume that he would appear in Marvel’s film universe at some point (especially given that he’s already on Disney’s payroll for ABC’s Castle). The notion of the aging actor playing a Marvel hero like Richard Rider a.k.a. Nova was always a bit of a stretch, but there was no question that James Gunn’s sense of humor and cosmic cast would be a strong fit for Fillion’s skills (giving standout performances in both Slither and Super).

For those who wish to maintain the surprise of seeing how Fillion has been worked into the plot of Guardians, now is the time to stop reading, and be content with Gunn’s confirmation that the actor would be appearing in a “fun” cameo. But those eager to know can read on.

It was Fillion himself who let it slip that his cameo may not be one that every moviegoer would recognize, claiming that viewers should “check the credits” to see where he had fit into the film’s cast. And according to a high-level source of Schmoes Know, that’s because the actor’s performance is, indeed, a small one – and won’t actually feature Fillion onscreen.

The role in question is actually that of Cosmo, the telepathic Russian space dog who has proved invaluable to the Guardians throughout their comic book history. Once the subject of a Soviet-era Russian space program, Cosmo was launched into Earth’s orbit only to drift off into the cosmos, eventually being mutated into the telepathically-gifted (and talking) head of security for the space station dubbed ‘Knowhere,’ a base belonging to The Collector (Benicio del Toro), a character teased in the mid-credits scene of Thor: The Dark World.

UPDATE: Fillion has denied that he will lend his voice to Cosmo in Guardians of the Galaxy. Here’s the actor’s Twitter post, on the matter:

Gunn essentially warned that the canine cosmonaut won’t be playing a large part this time around, but the confirmation of Cosmo is something fans have been hoping for since the first signs of Rocket Raccoon (close friend and animal comrade of Cosmo) being done justice began to appear online. If the rumors are true, fans may see the unique relationship between the Russian dog and Rocket Raccoon (Bradley Cooper) in live-action.

It seems more likely that Cosmo’s ability to speak will be the showcase of the character in such a small cameo, as opposed to his ability to fire psionic blasts or control the minds of those around him. But given Gunn’s unique brand of humor – and his decision to cast Fillion in the role, as opposed to a lesser-known star – anything is possible.

What do you think of this apparent reveal? Does Nathan Fillion putting on a Russian accent to play a psychic space dog seem like a role the actor was born to play, or would you rather have seen him appear in the flesh?

Are you convinced that the role is as small a cameo as Gunn has claimed, or would Marvel be wise to include Cosmo in future films (especially action sequences)? We would say that having fought alongside the likes of Adam Warlock and Silver Surfer have earned this pup some big screen attention, but we invite you to share your thoughts in the comments below.

Guardians of the Galaxy hits theaters on August 1st, 2014.

Follow Andrew on Twitter @andrew_dyce.

Source: Schmoes Know

Update source: Nathan Fillion