Usually, a person starts out with a single primary research interest as a PhD student (closely related to that of their adviser). But then as they move on through the assistant professor stage, they discover new research interests. Do they usually discover new research interests through collaborations/discussions with professors at their university or through conferences? Or do they discover new research interests through what can earn them funding (through the NSF and other agencies?) Especially since any topic could theoretically be interesting to them? (even though they might find some topics more interesting than other topics).
Is it more often that they do what they're most interested in, or that they do what they obtain funding for (and find what they get funding for interesting enough)? And can they sometimes even discover new interests through their PhD students?