Here are lines 63-69 of Act 2, Scene 1:
Between the acting of a dreadful thing
and the first motion, all the interim is
Like a phantasma, or a hideous dream.
The genius and the mortal instruments
Are then in council, and the state of a man,
Like to a little kingdom, suffers then
the nature of an insurrection.
The lines assert that, between doing something dreadful and planning it ("the first motion") - everything in between is like a hideous dream, a "phantasma". Man's faculties and abilities are in council - to decide the best course of action - and the "state" (of course, can mean the "nature" of something - the state of the weather, for example...) of a man undergoes a serious change.
There are lots of ways in which the ideas in this speech reflect conflicts within the play.
Firstly, what is the difference between doing something and thinking it. The assassination of Caesar might seem a good idea in the planning in the orchard scene (Brutus, you'll note, decides that the assassins rise against the "spirit of Caesar", which he claims has nothing to do with blood) - but the assassination seems rather difference when, after killing Caesar, there is blood all over the floor and chaos in the capital.
Secondly, the problem of controlling events: the way the body and the "state" of man reacts to the idea in the head shows the uncontrollable changes that can spring from an intellectual idea. In the same way, the "good idea" of the assassination produces all sorts of uncontrolled events in Rome.
And thirdly, the dream-like atmosphere of the play itself. Re-read the description of the supernatural storm, and think about the soothsayer and Caesar's ghost in Act 4. Might the whole play be a "phantasma"?
Brutus is torn between his love for Caesar and his love of principle:"Not that I lov'd Caesar, but that I lov'd Rome more"(III.ii.21-22). This inner torment is apparent in these lines from "Julius Caesar." Once he has decided to assassinate Caesar ("the dreadful thing"), the putting of the assassination plan into action involves great emotional turmoil, a conflict between the the desires of principle and his sense of self-righteousness, his affection for Caesar, and the reluctance that one feels to complete such a deadly deed. There is an "insurrection" against the conscience that cannot justify the act of "mortal instruments," murder even for the betterment of the state.
Later, after Brutus's and Mark Antony's speeches on Caesar, there is an insurrection in the hearts of the Romans, and Mark Antony becomes an enemy of Brutus. In Act V more conflict develops in Sardis between Cassius and Brutus as their argument centers around their honor, a word that rings of Mark Antony's eulogy over Caesar's dead body: "And Brutus is an honorable man...So are they all, all honorable men...."(III.ii.83-84). And, as Antony declared, judgment fled "And men...lost their reason! (III.ii.105-106). The State of Rome became worse than under the rule of Caesar.
I have a question also, "Would the phantasma or hideous be signified as what Brutus wishes the whole situation would be?" Could that be a correct statement or just an opinion?