A court in New York recently allowed a raunchy, one woman show relaying an alternative universe version of "How the Grinch Stole Christmas!" to go on in the face of a copyright claim by the Dr. Suess estate; with the reasoning that the use of the underlying work was "transformative". https://www.courthousenews.com/off-broadway-grinch-parody-defeats-copyright-claims/
The legal ruling is complex, and I will leave it to others to parse its validity; or whether the ruling will survive on appeal (if appealed.) I tend to lean towards the court's ruling, but I can argue the other side as well.
My point; you need to be very careful in advising that your client's use of materials that belong to others meets the Fair Use standard. Seldom will your client be using the work of others for artistic expression; they will usually want to use it for commercial promotion. In this setting, winning a Fair Use argument is usually a significant challenge.
Your clients will come up with multiple reasonas why their use is Fair Use. One argument I hear all the time is that de minimum use meets the "Fair Use" standards (i.e. the "if its under 6 seconds of a song it is alright" argument.) There is no such rule. While the amount of a work that is copied is relevant, it is not controlling. The analysis of whether the use is transformative has become the predominant issue.
You will also bring out the litigator in your client once you explain the "transformative" concept to them; your clients will immediately develop multiple arguments why their use is "transformative." In my experience the clients' positions are based on result based logic. Don't fall prey. Seldom is the proposed use actually transformative.
In the Grinch case, the materials are clearly parodying the underlying content. Parody is a great way to argue "Fair Use." But you'll find that your clients will misunderstand the difference between irony and parody. Using somebody else's work in a funny way is not parody. Parody requires that you be commenting on the underlying work itself; not contorting the underlying work to comment on something else all together.
The purpose of use is also critical. Arguing for Fair Use in an advertisement is difficult. In a critical article about the underlying work; much easier.
I've spent a lot of time with Fair Use; and other than in a documentary setting I usually will push my client away from the use. The analysis is seldom clear, and the cost of litigation will usually overwhelm the benefit of the use.
Just avoid using other peoples' copyrighted works without permission. The Grinch ruling is interesting; but applied in a very limited setting (an artistic work specifically questioning the artistic meaning of the underlying work.) For most of us, the use of someone else's copyrighted materials, sans clearance, is a losing proposition.