Post Production

In November 2013, Gunn stated that he attempted to use as many practical effects as possible while filming to aid the use of CGI and motion capture during post-production, saying, "Our sets are enormous. We have a prison that is 350,000 pounds of steel. Anybody who knows me knows I love the mix of practical and CGI effects... I can't wait for people to see it, because it's astonishingly beautiful." After the release of Thor: The Dark World, Feige stated that the Infinity Stones would be a focus in the film, as well as going forward into the Phase Three slate of films within the MCU.In a separate interview for The Dark World in November, Feige added that a third, unknown Infinity Stone would be seen in the film, referred to as the "Power Stone" by the Collector. The mid-credits scene inThe Dark World revealed Lovibond's role as the Collector's aide, later named Carina. In December 2013, Marvel confirmed that Diesel would voice Groot.

A few weeks of additional filming, involving the film's main cast and crew, occurred in March 2014 at Walt Disney Studios in Burbank, California.In April 2014, Gunn described Thanos as the "head of the snake" in the film, and confirmed he would appear via performance capture. In May 2014, Gunn stated that the film features an "enormous" amount of smaller and minor characters from the Marvel Universe, adding that he felt the film had the most characters overall of any Marvel Studios film to date.Costume supervisor Dan Grace added to this by saying, "We really, really get the feeling of the scale and scope of the galaxy. We visit five planets, we see a hundred different races." The film introduces the alien race Sakaaran, who act as Ronan's mercenaries, as a replacement to the Badoon, as the Badoon film rights belonged to 20th Century Fox.

By the end of May, Josh Brolin was revealed as the voice of Thanos, with Feige confirming in July that Brolin also provided the performance capture for the character.Thanos communicates via hologram for much of the film, though he does appear in a scene with Ronan and Nebula. In June 2014, Feige added that Thanos and his followers are "the biggest piece of connective tissue that will eventually lead us back into Avengers films in the future.On casting Brolin, Feige said,

We reached out to him and it was one of those things that does not happen all the time but when it does it’s very nice, where he was totally intrigued. He was a fan of what we did, he met with Jeremy Latcham in a hotel in London and learned about the characters a little bit. I spoke to him on the phone a few times. We ran it by James who loved it, ran it by Joss [Whedon] who loved it because Thanos is in this universe because of Avengers. Then we shot him and recorded for it.

On July 7, 2014, Gunn announced on social media that he had completed work on the film. In August, regarding the post-credit scene, Gunn revealed that the scene did not involve Howard the Duck when it was originally filmed, rather he was added during post-production a decision made by "some combination of [Gunn] and the editor Fred Raskin". As the decision to add the character was made late in the post-production process, he had to be designed that day, before being handed off to Sony Pictures Imageworks to animate.Also in August, regarding the pre-credit scene of Groot dancing, Gunn stated that he himself danced to provide motion reference for the animators, and that the decision was made to place the scene before the credits, rather than during or after them, because of positive responses from a test audience, which made Marvel and Gunn feel that they didn't want "people walking out and missing this thing".