As quoted by The Hollywood Reporter, Whedon calls the experience "a nightmare" and "the hardest juggling act I’ve ever, ever had to pull off." If you were in his shoes, you might feel the same way. He said,
They’re very disparate characters. The joy of the Avengers is they really don’t belong in the same room. It’s not like the X-Men who are all tortured by the same thing and have similar costumes. These guys are just all over the place. And so it’s tough.
It’s a pretty legit dilemma. Steve Rogers (Chris Evans) is coming off of witnessing the destruction of S.H.I.E.L.D. and searching for The Winter Soldier (Sebastian Stan), Thor (Chris Hemsworth) has relinquished his Asgardian throne and has been living on Earth, and Tony Stark (Robert Downey Jr.) is tired of playing savior to the world. In addition to all these standalone stories that we saw play out, Natasha Romanoff (Scarlett Johansson), Bruce Banner (Mark Ruffalo) and Clint Barton (Jeremy Renner) are grappling with their own demons. Add in all the new characters — Wanda (Elizabeth Olsen) and Pietro (Aaron Taylor-Johnson) Maximoff, Ultron (James Spader) and The Vision (Paul Bettany) — and you have one of the best reasons to become a little unhinged.
Plus, Whedon has not only directed both Avengers films, but he’s also had a hand in all the other films in between. He came in as a consultant to help director Alan Taylor on making sure Thor: The Dark World was cohesive with all the over-arching Marvel cinematic universe. He gave James Gunn notes on his take for Guardians of the Galaxy. And he directed the post-credits scene of Captain America 2 that featured Scarlet Witch and Quicksilver. No wonder Whedon is feeling the burn.
Did you see the new trailer yet?