He claimed he had no idea he would spend several weeks of his life in high heels and "encased in glue, latex, and a 40-pound suit." The Apocalypse suit turned to be so unwieldy that he could barely move inside of it, and even needed a "specially designed saddle" to be able to sit. And it must've been hard for his castmates to be convincingly scared of a terrifying immortal demigod when that demigod is constantly being ferried around by interns.
What seemed to irk Isaac most of all, though, was Apocalypse's terrible effect on his social life. One of the main reasons he was excited to join the X-Men roster was "to work with these great actors that I like so much." But because the suit made in the Batman style of not letting you turn your head, he was unable to even look at his co-stars during most scenes. Not only that, but the suit was also so excruciatingly hot that he needed to be rolled into a cooling tent between takes, missing out on all the terrific banter about Beast's blue balls.
Not that he even got to do a lot of great acting, in any case. Besides being goofy, unwieldy, and hot, the suit's constant loud squeaking made all of Isaac's audio unusable, and the actor had to dub his lines in afterward so that every scene with the intimidating monster didn't sound like Cyclops was trying to buff a dent out of the Blackbird right outside of the frame.
20th Century Fox