Let's see what you are seeing. If you are talking about filling between two separate pieces of wood, filler is not recommended in my opinion. What climate are you in? How were the window installed? Large swing in relative humidity via a poorly sealed connection to the home, will generate larger movement between the trim sections.

We're in Maryland, near DC. So hot and humid in the summer and usually not too cold in the winter. Don't know how to answer your question about how the windows were installed. Remodelers installed them. Here is a photo that shows the problem.

How they were "installed" pertains to how they were set. Full tear outs with flanges, full tear outs, etc.

Was this a new addition to the home? The large temp swings we are having right now can acerbate this issue and if the window are poorly sealed to the shell of the home, the humidity fluctuations will do this.

Yes, all of the affected windows are in both of two additions that were done more than 10 years ago, a sunroom on the back of the house and a partial second story on top. Not replacements. Whether they are well or poorly sealed, I guess I can't say for sure, other than to say that our heating and cooling bills are not out of the ordinary.

Only want to determine would be to pull the trim. That's a bit like cutting off your nose despite your face at this point. I would remove the old sealant and apply a good sealing in place of it. Repaint.

Sorry, I don't understand. What sealant?

There is likely caulk between the casing trim and jamb extensions. Those gaps are not "normally" covered with paint. If that's all that was used, that is likely part of your issue.

You have to understand and accept that wood moves. It expands and contracts in response to fluctuations in humidity.

The only real fix for this is trim replacement and proper installation methods, using reveals. Back prime/ prime end cuts. The wood is still going to move but the reveal gives you a place for caulk if you wish. Or not. The reveal hides/ disguises any crack or at the very least, makes the separation look logical. The separation won't be in your face in the way your current install is.

Thanks. That may well be the best way to go. But I'm not going to rip out the trim for 15 windows, most of which are 70 inches tall. I'm going to do the best I can with what I've got. So my question is, what is that? Caulk, wood filler, or something else?

Its caulk you use. Make sure its the type you can sand, and that its the type you can paint, but, also has some silicone in it. This is what you are looking for. Hope this helps. acrylic latex caulk

For interior window trim, use an acrylic latex caulk. Loctite Polyseamseal Acrylic Caulk with Silicone is a high-quality acrylic caulk with added silicone for durability and adhesion.

+1 to the above advice. Given that you won't be removing anything, I'd first make sure that the casing is secured (add nails if not), and then use some painters caulk on the seams. Filler is fine in nail holes etc, but not so great in seams that can have some movement.