Character relationships should come under strain over the course of your novel or screenplay so that tension in the relationship rises.  To accomplish this, you need to create dramatic moments in which outside pressures put each relationship in an increasing vice-grip.

Conversely, overemphasizing tension might be detrimental, especially in particular genres.  For example, in light comedy, action stories, and so on, relationship issues are not likely to be all that crucial or central.  Nonetheless, relationship stress should still rise, just not to the same depth and degree.  In short, keep an eye toward the overall mood you want for your story, and within that scope, bring tension to its maximum by the end of the third act.  

Tension does not have to rise smoothly, but can lurch forward in fits and starts.  !  The key is to mimic real life and the naturally uneven nature of the stress in our lives.  Tension can rise slowly, then drop quickly in a momentary release, only to begin to rise again.  Or, it can snap into place precipitously, only to gradually fade away.  In fact, a single relationship might employ both of these techniques.

No matter how you get there, you will want to eventually arrive at a set of dramatic circumstances that brings each relationship to the maximum stress level.  That is the point at which the relationship will stand or snap - the character climax of your story.