Being predictable is often seen as a negative thing. Yet predictability is tied up with what it is to be truly known by someone. When someone knows us well — and that is a precious thing — we become quite predictable to them. In fact, if our predictability is associated with good patterns of behaviour, it even adopts names like 'reliability' and 'faithfulness'.
But all the same, there are a number of of negative things about being predictable: first, when it becomes boring — a person remains steadfastly limited to one idea or area of interest, never expanding; second, when it is becomes stubbornness — a person doesn't change even when they ought to; and third, when it leads to being 'de-personised' or manipulated. This last one can happen even if the predictability isn't associated with anything bad (like in the first two cases). If someone uses a faithful trait of our personality to manipulate us for their own means, to treat us like a robot whose actions can be depended on and therefore can be exploited, we feel very insulted and if we detect it will often change our behaviour just to avoid it.
When it comes to knowing God, it seems that the situation is accentuated massively, and in both directions:
On the one hand, unlike us God is totally unchangeable, and has revealed to us many things about His immutable character — and therefore is ultimately predictable. I can predict, for instance, that God will never leave me, nor forsake me; that He will work all things for the good of those who love Him; that He will hear and save all those call on His name; and fulfil countless other promises. To doubt these things or fail to depend on them is in fact a great insult.
On the other hand, the danger of treating God as a 'known quantity' is massively increased. He is the one person who is ultimately unknowable in His actions, who cannot be boxed, and who alone is completely free to do as He pleases. To treat this One like a robot or a vending machine that can be manipulated according to our whims is both crazy and wicked.
One very practical way our understanding of this will be evident is in our prayers, and you can see both dangers in people's praying. We can often fail to take seriously the very great and wonderful promises of God — so, for example, a Christian might fail to come boldly to the throne of grace, when he or she should know that they are clothed in Christ's righteousness and cannot but be accepted by God. The other extreme, of attempting to use God's own character to twist His arm, so to speak, as you sometimes hear, is equally odious.
I think this is a difficult tightrope to walk, with perhaps no easy answers. One verse which springs to mind is where it says “there is forgiveness with You, that You may be feared”. Not “there is forgiveness with You, that we can sin all we like and know God will forgive us, 'that's His job'”. A true understanding of God's incomparable character and even His grace will produce humble, enraptured fear, and not presumption.