Working with the online example:
https://kitware.github.io/vtk-examples/site/Cxx/Utilities/DetermineActorType/
When I use the following, I don’t see that the font is drawn bold.
cubeAxesActor->GetLabelTextProperty(0)->SetBold(true);
I’ve tested this with various font and background colors. To my eyes, SetBold(true) has no effect.