Spoilers for said tag follow.
In the after-the-credits scene to Guardians of the Galaxy, we see del Tor's character, the Collector, conversing with Howard the Duck.
Director James Gunn reveals that this wasn't the plan from the outset, but rather something he came up with after filming.
"We did not shoot that scene to have Howard the Duck in it," explains Gunn. "It was something I thought of during post-production. I just thought we'd pushed the movie so far, why not push it all the way for the ending tag?"
Del Toro has seen the film since then, but never with the tag attached.
"Well," laughs Gunn, "shocking as it may appear, there is no actual Howard the Duck that we shot. He's CG. Benicio has seen the movie a bunch of times, and loves the movie, but we didn't have the tag after the credits until we released in the theaters. Benicio might know now that he's with Howard the Duck at the end of the movie. [But] I don't think he's ever seen the scene."
Guardians of the Galaxy is currently playing in theaters.