Behaviors vs Simple Scripted Validator

[Sales_Ops]

I want to make field B required when field A is not null. Field A and B are both write-in fields, and not drop-downs..

how do i do this with as little pain as possible?

[Ilya Turov]

Even though using behaviour here looks more flashy (basically, validating input data on the fly), I would recommend a scripted validator, since it's much easier to make a mistake while configuring said behaviour.

Plus, validator is tied to workflow, so you will always know why there's suddenly a required field.

There's plenty of documentation for writing a simple validator from adaptavist, but I believe your's would look close to this:

!cfValues["field A"] || cfValues["field B"]

meaning you either want field A to be empty or, when it's not empty, you want field B also to have something in it.