Dave Bautista, who plays Drax the Destroyer in the movie, said that he wanted this job more than anything else he's wanted in his life, auditioning 6-7 times for the part and "cried like a baby" when he finally got it.
Being a professional wrestler, he doesn't have much experience in more traditional film acting, and he admits it, but he said he tried his best to learn as much as he could every day on set.
When Vin Diesel started recording his lines for Groot, he was surprised to be reunited with the sound technician who he had worked with a long time ago when he recorded the titular character of The Iron Giant.
The fact that the movie got made at all is a testament to the faith Marvel has in their properties. It's EXACTLY the kind of thing they set out to do when creating their own production company.
From the second trailer: Groot giving a flower to a little girl.
From the Total Film BTS visit, Drax declaring the other Guardians to be his friends. Even the "green whore".
Similarly, the featurette introducing Gamora in which she says "I have lived most of my life surrounded by my enemies. I would be grateful to die among my friends."
From the 5-minute extended clip of the team are being incarcerated at the Kyln, Peter Quill after snarking back and forth with Rocket notices evidence of Rocket's experimentation and gives him a silent sympathetic glance.