Robinson's performance is quite good at realizing Caligula's character even though the film technically does not get into Caligula's personal lusts, in fact a different actor could have played him as just some jealous power hungry guy. That's not the case for Robinson who seems to suggest Caligula's various perversions merely in that devious grin of his and just the whole odd way he carries himself in every scene. Robinson seems to show a man who loves various pleasures far too much just in the way he slants in his chair. Honestly Robinson does not waste a gesture in making his Caligula one sick man even though we don't see much of it first hand. Technically speaking most of the time is spent with Caligula terrorizing Gallio eventual wife Diana (Jean Simmons) as he basically wants Gallio dead after he has converted to Christianity. Well even within the limits of the role Robinson still makes sure to get the most out of it.
Robinson is incredibly entertaining in the scenes where Caligula is in some way thwarted in his attempts to get Gallio. Robinson plays Caligula in these scenes like a spoiled brat who's not getting the present he wanted. Robinson is glorious in showing the rampant insanity of Caligula that in it's heart is childish in nature fitting for a man who has anything he wants handed to him no matter how crazy the request may be. My only problem with Caligula in this film is he's not in the film as much as Nero was in Quo Vadis. Robinson has about four scenes in the film and two of those scenes are a bit short. There's just not enough Caligula to go around because he's unquestionably the best part of the film. Robinson energizes the proceedings whenever he shows up and actually makes the film entertaining with his antics. Almost alone Robinson prevents the film from being completely forgettable as he gives one reason to watch the film.