So yes, fan favorite actor Nathan Fillion (
Firefly, Castle, Slither) will appear in Marvel's
Guardians Of The Galaxy..kinda! The Schmoes report that he'll actually be playing a motion capture character, and providing the voice. Well, unless they plan on sticking him in a dog costume that is..
"According our very reliable source with intimate knowledge of this and the production has spoken exclusively to us about this and we here at Schmoes Know have our answer: Nathan Fillion will be voicing the part of Cosmo, the psychic Russian cosmonaut dog that has appeared in the Guardians of the Galaxy comics."
Many of us guessed it'd be Cosmo, and there you have it. A fun little cameo I'm sure you'll agree.
UPDATE: A tweet from James Gunn casts some doubt on this, BUT, it's important to keep in mind that directors/actors/producers etc will frequently flat out lie about this sort of thing.