It took Joss Whedon a year to convince Aaron Taylor-Johnson to accept the role of Quicksilver. Johnson was concerned over the intensity of the Marvel contracts, the time constraints, and the fact that it was going to be such a large cast. Even after he accepted the role, he was still nervous, but was comforted after he learned that his friend and Godzilla (2014) co star Elizabeth Olsen would be playing his sister and would be his filming partner through most of the movie.
Because Ultron was described as 8-9 feet tall, James Spader had to wear an antennae-like contraption made out of a thick piece of wire with two red balls attached to the top that went up his entire back and 3 feet above his head. This was done so that the actors that shared scenes with him would be able to have a reference point for where his eyes would be (the two red balls represented the placement of Ultron’s eyes). Elizabeth Olsen stated that this was actually distracting because Spader would be giving an intense performance and out of instinct she would look at him rather than the balls representing his eyes. Much to everyone’s amusement, whenever this happened, Aaron Taylor-Johnson would yell, "Red balls! Look at his balls, Lizzie!" at her in order to get her to look in the right direction.