This is an instance in Julius Caesar where Shakespeare leaves us wondering some about what Antony thinks in full about Cassius. Caesar's last line in Act I, scene ii before exiting with Antony is, "And tell me truly what thou think'st of him." The subject never comes up in the text again.
Antony might have--but didn't--lingered to deliver a soliloquy revealing what he "truly" thinks of Cassius. Apparently Caesar was not satisfied that Antony had spoken candidly (openly, without restraint) the first time he answered, but that answer is all the audience/reader has to judge by.
At Line 1.2.190 (Act I, scene ii, Line 190) Caesar calls Antony over to him then requests that Cassius be replaced in the retinue surrounding Caesar because Cassius has "a lean and hungry look" that Caesar thinks marks him as too much of a dangerous, greedy, ambitious man to have around. Antony replies by saying,
"Fear him not, Caesar; he's not dangerous;
He is a noble Roman and well given"
["well given" means well disposed]
Antony's opinion that Cassius is "not dangerous," "nble" and "well given" is insufficient for Caesar as he goes on to say that Cassius is too focused on observing others, too serious-minded, too prone to smiling in mockery and too interested in greatness for themselves and therefore dangerous.
After this he asks Antony to speak his true opinion. Following which, as said before, they both exit. This leaves us knowing far more about what Caesar thinks about Cassius than what Antony thinks.
But we can infer that Antony isn't the suspicious sort nor the sort who looks about in jealousy or greed, as a result, he seems to have seriously misjudged Cassius because even as Antony is asked his opinion, Cassius is weaving the threads of a plot against Caesar's power...and Antony likes the fella and perceives no danger or other ill-disposed character traits in him.
[For additonal notes on Julius Caesar, see Shakespeare Navigator at ClickNotes.com.]