It's not just about having the power...

[Legate of Amon Lanc]

Originally Posted by Ibrîniðilpathânezel

It's fairly clear (at least to me) that in Tolkien's mind, Saruman was not secure in his own power, and was afraid that sooner or later, Gandalf would move against him. His fear of Gandalf is mentioned again, as is his eventual knowledge that Gandalf possessed one of the Elven Rings. I think that at the time he imprisoned Gandalf, Saruman didn't dare try to start anything he feared he could not finish victoriously. If he had felt confident of his ability to defeat Gandalf in a battle of power, I suspect he would have tried to at least wrest Narya from him (since by this point, he's certainly aware that Gandalf has it), to presumably weaken him and put Saruman in a better position to force information about the Ring from him. But he doesn't. He tries persuasion and intimidation, but both ring hollow, as does his attitude of superiority. Saruman talks the talk, but he doesn't walk the walk. He has armies and guards to defend him, but he doesn't do more than imprison Gandalf. Saruman's afraid of him, a fear compounded by paranoia.

Indeed. It's this typical "the one who pretends something is always fearing when somebody might reveal him"-behavior. As is well illustrated on the example with Saruman's secret smoking, even this was a vain fear, because for example in this case Gandalf knew, but did not wish to tell anybody, "for it was never his wish that any one should be put to shame". Of course, concerning more important matters like his treachery, this was something different - but the example above goes with what Ibri said about different projection of Gandalf in Saruman's mind and the real Gandalf.

However, what you say, is indeed worth attention, and thanks for brining it up. It seems indeed, when I think of it, that imprisoning Gandalf was far more of a loss to Saruman than anything else (of course not speaking now about the consequences it had later). So, here I would emphasise once again, the more now: the fact that Gandalf did not fight back was not a loss, and by no means a capitulation. I would now dare to say explicitely: it was a victory for him, and a loss for Saruman.

What does Saruman do? He:

1. does not get an information about the Ring from Gandalf, because Gandalf refuses
2. does not get his support, because Gandalf refuses
3. does not take Gandalf's staff, because he does not feel up to it (probably he is afraid that touching the staff, as the attribute of power, might push Gandalf to fight Saruman, of which Saruman is afraid - again, this is Saruman's projection of Gandalf; I think Gandalf probably won't attack in that case, the power - or the staff - is not that important to him to threaten Saruman's life)
4. does not take Gandalf's ring, the same as above
5. can do only one thing, the only one to which he can resort now, he locks Gandalf up. Which, if you think of it, is a pretty cowardly thing inside, although of course Saruman was "playing the cool one", but was unsure inside (also because probably he was afraid of Sauron getting the Ring, too, despite his high talk he was uncertain what would the Lord of the Rings do with his dirty little wizard fellow when he has the power back. Traitors are always afraid - although this time probably rightfully).

There's one thing, that I am not sure how much Gandalf was aware of his own superiority to Saruman in this aspect. He was still in the state of not knowing well what he's dealing with, grave news coming one after another, and not sure how far Saruman's treachery reaches. I think the self-confidence brought by the knowledge of how the things are (for example seeing better into Saruman's mind) was one of the things Gandalf gained with his return as the White Rider. Now the more I think about it, wasn't it really mostly about this knowledge, more than any "new special powers"? I think when we consider what's said in the books, it might as well be like that, maybe besides the "new body"... But this would be maybe for another thread, wouldn't it?
Anyway, Gandalf remained certain in the things that he won't give up to Saruman and that he won't resort to harming him. And had he been aware of his superiority, now the more important would the choice of not going against Saruman be, again the ultimate: having the power (and really an absolutely superior power, and being aware of it), AND not using it.