In Julius Caesar, if you could ask Mark Antony and Decius Brutus a question, what would it be?

I would ask Mark Antony and Decius Brutus if they knew what was in Caesar’s will.

Mark Antony and Decius Brutus were both lieutenants of Julius Caesar.  In fact, you could say that they were the two closest men to Caesar.  Either one could have been named Caesar’s heir, but instead he chose Octavius.  If either man had known, it would give them motive for killing Caesar because they felt unappreciated.

If I were investigating Caesar’s death, the first question I would ask each of these men is whether they knew what was in Caesar’s will.  Although we do not know if Antony was involved in his death, it is possible that he was.  We know that Decius was heavily involved.  He made sure that Caesar made it to the capital so they could kill him.

Although there were many involved in the conspiracy to assassinate Caesar, Decius Brutus was in it up to his neck.  Artemidorus tries to warn Caesar that “Decius Brutus loves thee not” (Act 2, Scene 3).  Decius Brutus was given the very important job of ensuring that Caesar was there.  He did it admirably.  Caesar trusted Decius so much that the man was able to talk him into going to the senate even when he wanted to stay away.  Decius reinterprets Calpurnia’s dream in a convincing way.

This dream is all amiss interpreted;
It was a vision fair and fortunate:
Your statue spouting blood in many pipes,
In which so many smiling Romans bathed,
Signifies that from you great Rome shall suck
Reviving blood, and that great men shall press
For tinctures, stains, relics and cognizance. (Act 2, Scene 2)

Caesar trusted Decius and respected his advice.  He had no reason to suspect that his second in command was leading him into a trap.

Although we know Decius Brutus was involved, there is no evidence that Antony was not.

Antony immediately begins to consolidate power after Caesar’s death.  He makes sure Octavius does not enter the city, and he has Caesar’s will very quickly.  He knows that he can use it to win over the people.

It is not meet you know how Caesar loved you.
You are not wood, you are not stones, but men;
And, being men, bearing the will of Caesar,
It will inflame you, it will make you mad:
'Tis good you know not that you are his heirs (Act 3, Scene 2)

Antony makes no mention of Octavius, Caesar’s new heir.  Caesar posthumously adopted Octavius.  Antony was older, and a close relative as well.  He had served Caesar for years.  Why would Caesar choose Octavius over him?  Caesar knew Antony’s personality, and might have doubted his loyalty too.

Why would these two men turn on Caesar?  By all accounts they were faithful, high-ranking military men that Caesar would have trusted with his life.  Shakespeare gives no evidence that Antony betrayed Caesar, but he could have.  He was absent at the crucial moment.  Was he really tricked into leaving by Trebonius?  Perhaps his anger and grief in response to seeing Caesar’s body was not genuine, but an act to make sure no one suspected him of being involved in the conspiracy.

Historically, we do have some evidence that Decius did approach Antony and ask him to join the conspiracy.  What we don’t know is whether he did betray Caesar or not.  Some historians believe that both men knew that they were shortchanged by Caesar’s will.  They would have seen Caesar naming a young upstart like Octavius as a serious affront.