(How I did it is a little complicated, but rest assured it didn't involve necromancy, reanimation or time travel.)
They all mostly wanted to gloat about their later careers, but I tried to keep them to the subject at hand. "Why'd you do it? Some say it was to stop him from becoming King, but that didn't pan out - his adopted son became Emperor instead. So, what gives?"
They seemed embarassed about it. "Senatorial peer pressure, mostly," said one, "And I wanted to look cool."
"It was part of a deal," said Cassius, "I had to get in on the assassination as a tit-for-tat, to get votes for a reduction of taxes on linens."
"I wanted to be like my favorite gladiator," said Casca, "Feint! Stab! Thrust! I looked so awesome."
"I was going through a rebellious phase," said Brutus, "and I was jealous of Julius. He got all the chicks."
"I thought we were stabbing Pompey," said another apologetically, "And I couldn't push my way past the crowd."
"No comment. I have no idea what you're talking about," said another, "Talk to my lawyer."
I don't know why I bother.