The answer depends on the parties involved and the property. At the outset, one normally communicates with the person or entity from whom the rights were obtained (which, depending on the level of the production, might be the author's agent or lawyer, or a licensing agency) but there may be direct contact with the author if there is a relationship. Whether a request is granted likewise depends on the production, the author, and the change. A high profile production with people with credibility will often get license that will be denied to someone lacking any provenance. But there are also times that small productions might get permissions that would be denied to, say, a Broadway production. Also, there are authors who grant reasonable changes fairly liberally and others who never do. I would say that in absolute terms changes are granted pretty often, but the overwhelming majority of them are very small. As with most things, there are no rules.