Risks and business conditions change all the time, so an annual plan or even one that is updated quarterly won’t lead to auditing what matters today. You audit what used to matter.
In Auditing that matters (which I strongly recommend for every internal audit practitioner or consultant), I have a chapter on making sure you have the audit team you need. Here is an excerpt from the start of that chapter. It talks about probably the most important skill I needed from every member of my team.
The majority of internal audit functions perform a variety of audits every year and provide an opinion (ideally) or at least a list of risk-ranked weaknesses (far less than ideally) on the scope of each audit.